I was talking to someone who said they were treated quite well. He cited a abolitionists journal who went down south or something.
Well, let's be clear from the outset: your friend is very, very wrong.
The idea that slaves can be treated well in any meaningful way is profoundly misleading one - there is nothing about the condition of slavery which can be described as good. We can certainly say that one slave is treated more or less severely than another but a slave is a slave; the entire construction of racial slavery in the United States depended upon the systematic and relentless degradation of the Humanity of the enslaved.
Slavery is fundamentally a relationship of total domination between slave and master. A slave is literally the physical property of his or her master, without condition or limitations to be imposed on the extent of that ownership. This was not merely a legal judgement but a social reality: the dependence of a slave on his or her master to function within society is total. Consider, for instance, this extract from a court ruling in 1829:
With slavery [...] the end is the profit of the master, his security and the public safety; the subject, one doomed in his own person, and his posterity, to live without knowledge, and without the capacity to make anything his own, and to toil that another might reap his fruits [...] such services can only be expected from one who has no will of his own; who surrenders his will in implicit obedience in the consequence only of uncontrolled authority over the body. There is nothing else which which can operate to produce the effect. The power of the Master must be absolute, to render the submission of the slave perfect.
These are the words of Thomas Ruffin, a senior judge in North Carolina. Ruffin wasn't criticising the nature of slavery or arguing for its abolition; he was dispassionately explaining the legal and social reality of slavery in a high court judgement where he found that it was perfectly legitimate for an owner to seriously injure his slave in order to ensure obedience. If we venture across that state's border into South Carolina, we can examine the Slave Code - the law governing the treatment of slaves - that existed there from 1740. It explicitly states that "whipping or beating with a horse whip, cow-skin, switch or small stick, or by putting irons on, or confining or imprisoning [a] slave" are all legally permissible without penalty or condemnation, whilst various even more serious offences (including castration, amputation, blinding and burning) incurred only a minor fine; even outright murder was only punishable by a fine, albeit a rather more hefty one. Such laws existed throughout the South and represented the codification of established practices.
To be a slave in the antebellum South meant to exist in a condition of the most incredible physical and psychological vulnerability. If you were born in Africa and taken to the Caribbean or the United States, then you would have endured the horrific experience of being kidnapped, transported in some of the most degrading conditions possible (malnutrition, disease, torture, murder and suicide would claim the lives of at least 1.8million men, women and children on the journey to the New World), taken to a strange land where you had no concept of society or language and sold to someone who could do effectively whatever they wanted to you. If you were born in the United States, then from birth you were raised with no doubts about your status or condition; to simply allow someone to be raised as a slave is a form of gross psychological abuse. You would be fortunate beyond belief to enjoy even a rudimentary education; for most of the period, it was illegal even for a white owner to teach his slaves to read. Even as a child, you would be expected to labour, assuming you survived to adulthood - many children in the Lower South perished simply because their parents couldn't look after them whilst also meeting the punishing work schedule of the southern cotton plantations.
Even family life was not sacrosanct. Slaves were property, one and all; bonds of kinship were irrelevant within that paradigm. Parents, children, siblings, spouses - especially in the Upper South, you could be sold away from all these people in a heartbeat. Wherever you may have ended up, you may not have even had the freedom or autonomy to choose a partner of your own. Rights to marriage and family life were rights people, particularly Christian people, were entitled to; slaves were not afforded such respect even for these basic dignities.
So I would argue in the strongest possible terms that you could not treat a slave well at all. As a white slave owner, the only way to treat a slave well would be to liberate them; otherwise they were still part of the psychologically and physically degrading system of slavery. The only reason to keep a slave, save perhaps in some extraordinary circumstances (like in states where laws made manumission difficult or impossible), would be to extract some kind of benefit from that system of systematic, relentless violent degradation.
Now certainly within that paradigm, some slaves were treated more favourably than others. Some were afforded privilege or status that, in material terms, allowed them to live comparatively decent lives - though such individuals were really few and far between in the grand scheme of things. Others may have had owners who were disinclined to the use of violence in all but extraordinary circumstances, or who may have been unusually generous in their treatment of slaves. Conditions could vary considerably from slave-owner to slave-owner, and certainly slaves themselves often expressed the view that there was a hierarchy of severity between slave-owners; many in the Upper South for instance rightly feared the prospect of being sold to the Lower South, where massive plantation operations tended to lead to harsher discipline, poorer working conditions and less interest in the day to day wellbeing of individual slaves. Anywhere in the USA though you would struggle, however, to find a slave who had not at some point in their life experienced what we would consider abhorrent, degrading, violent treatment at the hands of an owner. Even slaves who enjoyed exceptionally good treatment knew well that their situation was precarious and that they were, legally and socially, incredibly vulnerable and not seen as fully Human.
It should be said that there is quite a debate about the extent to which slave owners took an interest in their Human property's well-being. That they could do and often did from the 18th Century onwards is apparent, but their motivations are not. It has been argued (mostly famously by Eugene Genovese) that an ideology of 'paternalism' developed among slaveholders across the South, in which, troubled by the obvious Humanity of their property, slave owners sought to Humanise slavery by framing it in terms of a kind of parental relationship. The paternalist model sees efforts like Christianisation, provision of healthcare and food and the like as attempts by the plantation class to civilise and care for their slaves. It should be stressed however that such an ideology still depended upon racial degradation; upon the idea that black people were inherently inferior to whites and thus slavery was a means by which they could be given purpose and motivation, with whites reaping the profits of their labour almost as a kind of compensation for providing care to these inferior people.
Critics of this model are inclined to the view that Christianisation and general attempts to care for slaves were motivated purely by economic self-interest; a slave was a significant and expensive investment, and providing for their wellbeing maximised their working lifespan and thus profitability. Christianisation can be seen as a means of social control by constructing a religious philosophy that tried to make slaves accept their lot in life as ordained by God (and certainly, slave owner-sponsored Christian preachers were preaching a rather selective gospel!). Paternalistic language and rhetoric can arguably be seen merely as a means by which this economic self-interest was disguised as altruism, as reflected in the fact most owners were still more than happy to continue using violent punishment or breaking up slave families. Arguments can be made that paternalists over-emphasise the more benign aspects of slavery in day-to-day life at the expense of significant acts of degradation that, whilst less frequent (no-one can work if they're being beaten half to death every day), ultimately define the experience of slavery much better. Certainly black recollections and documentation of the slave experience focus much more on these.
For reading suggestions: