I'm going to side with your GF. If you analyse this logically, the facts don't add up.

Firstly, your GF chose her moral integrity over her friendship. Does this sound like a person who would cheat? I admit that morality is a poor predictor of human behaviour, but I thought this was worth mentioning nonetheless.

The instinct of self-perseverance, on the other hand, is a much better predictor of human behaviour. Let's, for argument's sake, assume that your GF did in fact cheat on you. Given your GF's friend's claims, I assume that your GF would have known that her friend was aware of her betrayal to you. Now, I ask you: would someone who (1) knew of their friend's potential for retribution, and (2) lack the character to remain loyal in a relationship been driven by a sense of right and wrong to come clean about her friend's indiscretions?