IMO she cheated on you... I've seen exactly the same thing happen to a mate of mine. It's not nice, but it may be worth thinking about how much you can trust her...
Trust cannot always be easily assessed any time, right now from the sound of it, his girlfriend is in a bad way, its not a case of cheating and feeling bad, cheating and no remorse or anything like that, she's obviously mentally distraughty by something that has happened, the OP has said how much he cares about her and wants things to work, before he can really assess the situation on a proper, level headed standpoint, he really needs to do as advised, try and get her to see someone to talk about the issues, with him if thats what she wants, and he should see someone as things like this can affect the partnering male too, not to the same degree, but certainly pulls at different heart strings. Once she's back on track to having a normal life, it'll be easier for them to see if they are meant to be or not.
Again mate, best of luck to you in all this, to you both.