I got my two boys from the adoption center and they were best friends in the shop, no humping at all and no signs of dominant behaviour. An hour after getting them home they started chasing each other, pinning each other down and pulling out chunks of fur. It was a bit of a shock to say the least because they were my first Rabbits! I'm guessing it was down to them being in a new/bigger environment. We kept a very close eye on them (we probably should have separated them really) and once they were neutered they went back to being best buddies!
Hopefully p@h at least got the sexing right and your cousin doesn't end up with any unwanted litters!