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.
don't forget that he also needs to account for a proper motherboard, both Intel and AMD offer similar MB with similar features but Intel may be slightly more expensive. If there will be no overclocking involved then Intel would be the way to go, if tweaking and OCing peaks his interest AMD would be my choice.