Oldest one I boot up regularly is Super Mario World I would say. On a less frequent occasion I do also play Tecmo’s Deception (PS1). I do like going back to Gradius for the SNES as well.
Contra and Contra 2 are fun, but very challenging games I will occasionally bust out.
PC gaming wise I have been playing City of Heroes and Diablo 1.
I would say unless you always want to do troubleshooting for this friend just stick him with windows. At some point it is up to the individual to be able to troubleshoot these issues using the Internet as the resource, but a lot of people just don’t want to mess with that. My own time is too limited to be on call for people.