Many HP computers are built specially to that company's design and are difficult to upgrade. You don't say how old the computer is but I guess it's not really worth spending money to upgrade
He would be much better advised to buy new.
If he has a good monitor he can always buy computer without monitor to save £50/£60
I suggest current offers from Dell (bearing in mind these can also be difficult to upgrade later) or Mesh
The main question is, what does he expect the computer to do?