If he has 'used it' for trial purposes only and not registered it (used the key) then you MIGHT be able to use it and register it - but you will need to check this out BEFORE you part money (he should understand this!)
If, on the other hand, he as registered it on his PC then, he needs to phone Microsoft and tell them he wants to install it on another PC (yours, though don't tell them that!). If they allow this then they will probably give him another key and he will be required to uninstall the original install on his PC. You then need to install it on your PC with whatever key MS gave. ONLY THEN, once all is ok, should you part with your cash. Again, he should understand this.