If he is using what would be an x300, x550, or x1050, then chances are he is still on the AGP bus and there are not new upgrades for him.
Still minecraft can be really intense. I built my brother a super cheap computer.
Intel Celeron G530 (Dual core 2.4Ghz)
2x4GB RAM (left over from my personal build or he would have 4GB)
Nvidia 430 GT
This build was extremely cheap, and he is able to play minecraft on it, but at several points he gets a lot of lag. The graphics card is actually sufficient but his processor cant handle it always. If you have any CPU less than this I doubt you can play minecraft.
I honestly think on the AGP bus you could probably play it with an x800pro. I am able to play Fallout 3 with my old pc using it, so you might get by with minecraft on it, but anything less than that I am positive you will not be able to. Even if the x800pro makes it, you still won't run great.