In my opinion:
It is totally up to himself if he wants to use it on singleplayer. But if he starts doing it on a server, then that might be a problem for the other players. Yes, maybe the only one he is cheating, is himself (and other players on a server). Nearly the same thing as spawning in items now and then, using NEI. But if he is on singleplayer, he can use that mod as much as he wants to.