Spaces sims style games are pretty few and far between, but Star Point Gemini 2 is decent, especially after the recent DLC.
Some people will swear by the X series, but I could personally never get into them, same goes for Evochron Mercenary.
Want something more reality based but still fun, Kerbal Space Program is a blast. It's the best crash course in orbital mechanics you can get, and I literally mean crash course.
For 2D stuff but both SPAZ and FTL are excellent 2D Sci-Fi space games.
And there is no shortage of Sci-Fi Shooters about, it all depends on what genre you friend wants to play.