It really depends on the mods. This is where you're going to have to do some independent research. Some mods are poorly coded and are resource hogs. Others, not so much.
Mods will eat up alot of RAM(the regular kind) and more importantly alot of VRAM.
Research your mods, pick out the best video card your friend can afford. Keep in mind since he's upgrading his CPU, this will most likely require a RAM upgrade and of course a motherboard upgrade. This will eat up a significant portion of his budget.
Also note, the next GTA is due out this fall... seems a waste to build a system around a game that's about to be replaced.