This isn't enough information to go off of. If they start casting hellfire at 5% health, or they use it when not healing them would kill the tank, then its their fault. In any other situation I can think of it is the healers fault. Also the healer should be healing them between fights.
EDIT: also as has been said, it seems some healers don't know what Hellfire is legitimate and if the lock is specced right then it is good dps. Healers should really consider things like this, you may think you are weeding out bad players when you are really punishing them for doing the right thing, which makes you look like a douche.