Overall performance wise, the i3 2100 is a better performer but cannot be overclocked. The FX-4100 is not bad and can be overclocked.
It really depends on each situation. The FX-4100 is almost a quad core, its 2 modules each giving about 180% of a dual core, while the i3-2100 is a dual core with SMT making it effectivley a quad core, but only about 20% effective to that point and most games do not benefit from the SMT in any way.
If he goes the AMD route, it would be better if he bought a Phenom II X4 or X6, as they will perform better than the FX-4100 and overclock as well.