Or it could be he thought it was totally innocent and doesn't password protect his computer... or left it on and she just looked...

Regardless, obviously he didn't think it was cheating. For all we know, it wasn't his girlfriend. Even if it was... maybe she also thinks reading Maxim or Playboy would be cheating. Many do.

You know... just because you have a partner doesn't mean you can't have separate friends and interests.