Can I just ask about this "other guy", seen as you saw the pics he put on Facebook do you actually know him in real life then?
Either way I'd consider simply removing him or even better blocking him on Facebook. I've been there before a while ago where I was jealous of another guy and was almost stalking every activity he did on FB as some clue to him being after my girlfriend. Trust me it doesn't lead to good things, and its annoying "not knowing" but maybe that's better than "assuming."
It all sucks, and as others have said whatever she decides then its her decision. I got screwed over by a girl once and was feeling crappy for a while, but some simple words my friend said stuck in my head over and over and made me feel better- "if that's the kind of person she is and that's the kind of things she does, then you're better off without."