The MAS doesn't allow you because you need the original apple id account and password.
Also that can be a problem if you need to reinstall the OS,i suppose that it come with Lion ...
ALways you will need the apple id to do software updates for ilife and may for Lion.
The guy was silly because he know that but didn't talk about it.
Ask him if he already link the laptop to him apple id because if no you can do it to yours,i am no sure if after link it you can transfer it but i don't think apple allow it,is like transfer music from one apple id to other it doesn't allow it so with software why different?
Be sure if he linked or no and if i yes be sure you can update the OS without a password.
May contacting apple and explaining the problem you can get some advice.