Ok the people who are saying "she definitely cheated" are really not worth listening to, obviously no one knows for sure if she cheated and I doubt anyone ever will. If neither person can remember what happened then you are NEVER going to get the full story because neither are going to remember what happened if they can't now. This is something you can either believe and move past or you can sit and dwell on it but the fact is that you won't get a straight answer if neither can remember what happened.
I have been in situations where I have blacked out from too much alcohol and not had recollections of what happened that night. It is really scary. I don't remember what happened some nights (I have had memory loss from drinking about 4 times in my life). But when people are drunk they USUALLY still know what they are doing and still contain their morals, or I do at least.
So I would say, believe what your gf says. If both her and the guy are known have morals then I would believe that nothing happened, esp since they were fully clothed. They were at a party and crashed out in the same bed. I doubt anything happened at all.
Your gf is obviously upset by what has happened and hates that she can't remember anything. At the end of the day, you can dwell on it for ages and not get to the bottom of it and break up with her or you can try and move past it and accept what both your gf and the guy say. I would go with the latter.