One of my friends insist that it was a major part of 'viking code' to never attack an enemy from behind. Do we know if this is true or not?
Hello, I wrote a longer answer here about the wider Danish military track record in England that I hope might prove useful context, but based on Danish strategy during the invasions of England, the answer to your question is "almost certainly not".
Recent pop-culture has been overwhelmed by a vast amount of - for want of a more academic phrase - neo-Pagan fanwank about "The Vikings" as these invincible, honourable bezerkers, when that really isn't the case. It's well-attested in contemporary sources that the Danish strategy in England - and elsewhere - is to avoid a fair fight as much as possible. Apart from its obvious material benefits, the Danes used raiding as a strategy before the institution of the burghal network to tie down vast amounts of English manpower, to pick off isolated, small garrisons, degrade morale, damage the economy and generally hamper the ability of the English kingdoms to face the Danes in anything resembling a fair fight, as these have a habit of going badly for the Danes. Broadly speaking, as a strategy, this directly encourages the attacking of an enemy from behind.