I've never met anyone personally who would even dare deny that the Holocaust happened.
That said, I understand the logic (or conspicuous lack thereof) behind claiming it was all a conspiracy with doctored photos and falsified documents. Is there anything a modern historian can do to break through the conspiracy theorist mindset, and convince someone that the past very much did happen?
/u/commiespaceinvader wrote a very excellent post on Holocaust denial and how to combat it.
Recent statistics coming out of Britain are suggesting about 1 in 20 people hold some form of Holocaust denying, or Holocaust obfuscating/revisionist beliefs. The standard statistically on antisemitism generally is the ADL Global 100 survey from 2015, however antisemitic acts have surged somewhere upwards of 60% in the years since 2015 so these stats are somewhat outdated. This survey also did not look specifically at Holocaust denial, rather asking "do you believe Jews still talk too much about what happened to them in the Holocaust" which they found about 20% of Americans believe. The ADL does however have statistics on Holocaust denial, suggesting that more than half of people world wide do not know about the Holocaust, which may leave them vulnerable to deniers, and that about a third of those who had heard of it believe it to be either exaggerated, or a myth. Multiple surveys conducted in the last year in Europe (and the UK) and in North America show both rising antisemitism (both violent and otherwise), rising Holocaust denial, and a poor grasp of basic facts of the Holocaust. This of course leaves people vulnerable to the conspiracy theories of Holocaust deniers. Denial is perhaps most surprisingly a growing problem in Eastern Europe, where most of the Holocaust happened, but as of 2015 almost one quarter of Eastern Europeans believed the Holocaust to be either exaggerated or a myth. In the same region there is increasing pressure on scholars of the Holocaust not to study exactly how the Holocaust happened in these countries, especially Poland, with particular pressure to avoid studying collaboration of local non-Jewish populations, which most scholars suspect is much higher than was previously known.
As for pushing against the antisemitism that is Holocaust denial I agree with the post referenced in the first post. Always Holocaust denial as the extremist ideology it is. Treat is as you would any other racist ideology. Do not debate the facts, but do try to reach the person through conversation if it is safe to do so. And if someone you know is falling into antisemitism or Holocaust denial, contact groups like Life after Hate which work specifically to deradicalize white supremacists and other extremists who tend to believe in Holocaust denial.
http://global100.adl.org/info/holocaust_info
https://sites.hks.harvard.edu/ocpa/pdf/HolocaustDenialPAE.pdf