How I see it is there is two ways it could happen
#1: he says "Hey guys! I can play Raining Blood on Expert in GH" if you put him down for that than you were being an asshole
#2: he says "I'm so good at GH I can play Raining Blood on expert, I bet I would be able to do it perfect on a real guitar now!" in this case it was him who was being an asshole
Of course the wording of these could vary quite a bit.
#1: he says "Hey guys! I can play Raining Blood on Expert in GH" if you put him down for that than you were being an asshole
#2: he says "I'm so good at GH I can play Raining Blood on expert, I bet I would be able to do it perfect on a real guitar now!" in this case it was him who was being an asshole
Of course the wording of these could vary quite a bit.