all depending on what kind of game he'd like to make.
Here are some good options:
If he wants to make a 3D game:
1) Unity 3D (not free, but THE BEST)
2) Irrlicht Engine (free, need to program in c)
3) Sauerbraten (good for mapping, code can be modified to make own game)
4) DarkBasic (don't know much about this)
If he wants a 2D game:
1) Flash (not free, need to program)
2) Game Maker (trial, not free)
3) Allegro (free, need to program in c)
4) Multimedia Fusion (not free, for people who are too lazy to program)
5) QBasic (free, good for classic dos games, need to program in basic)
6) AGS (if he's familiar with the classic Sierra quest games)