I'm guessing your friend is trying to lock his XP at 55 in order to get the full Ilvl 70 DK gear?
with that being said, the only way to do that now is if you had an old trial DK that never gained XP from completing quests in the starter zone, and reactivated that account. I have a few DK's that are level 55 with 0 xp and a few starter quests complete, however, it still might be impossible to stay 55 though even with a full group splitting XP. Gl though!