Let's say that you've just significantly upgraded your GPU. If you were getting the most out of your CPU with your previous GPU, there's a good chance that your new GPU will be held back by that older component. So now, you need a new CPU or some percentage of your new GPU's performance is wasted. Except, getting a new CPU that's worth the upgrade usually means getting a new motherboard, which might also require new RAM, and so on.
This guy's friends should keep him away from computers and just give him an iPad to play with.