Without checking, I would assume he probably has around a 300W generic PSU. In this case, the best he can probably do without also upgrading the PSU is the GTX 750Ti -- as long as it's one that doesn't require PCIe power connections.
However...without knowing the rest of the specs, it's impossible to tell if there's a better GPU that would go well with his CPU & RAM combination, as well as whether he has a decent PSU or should invest in a better one (which would also allow for a better GPU).