How good of a PC does my friend need?

I just bought the game for my friend, on steam. He's able to run the game on his laptop, at the lowest setting. And he's expressed that he wants to keep playing with my group, but he also hates having the game on that setting.

I've decided that I'm going to buy him a proper computer. And I want to know how good I need to make it.

Will a budget build work? Or will I have to go expensive?