It is true that Photoshop is superior over GIMP, but, program does not act upon itself.
Pro with GIMP would still be better than noob with PS..
So if he is comfortable with GIMP, let him stay with GIMP, then when time comes and he senses the need that he wants something more powerful he can switch to PS.
There is no need to yell at people how GIMP sucks and things like that.
And GIMP is freeware, while PS is not.
Hate will eat you up from the inside, so hurry up and give your heart to a girl before you get eaten up.