Sometimes I feel like mumsnet exists in two parallel universes or something. The number of threads giving women advice on gathering evidence about their cheating partners, LTB, etc. etc... and yet the advice here is "she'll find out soon enough"?!
Are we forgetting that the girlfriend is also a woman, who is entirely innocent in this, and is being cheated on by a scumbag? Do we not care because that isn't the woman posting here? I really, really don't get it.
Of course you should bloody tell her! Put yourself in her shoes. Would you want to be the idiot who doesn't have a clue what's going on behind your back? I highly doubt it.
So what if he denies it? Men deny cheating all the time- should we just not bother confronting them? 'Oh, he'll deny it, so I might as well just ignore it"- WTF? OP has clear evidence.
The poor girlfriend deserves a shred of decency here. She's currently being screwed over by both of you- do the right thing and let her know how he feels about her relationship.
I'll be honest- I have a close friend who was cheated on countless times by her ex. She then continued to sleep with them after they broke up and he had a new girlfriend. She kept doing it because it made her feel good- she felt like at last she was the one in the position of power and she felt flattered that he wanted to sleep with her. It fucked her up massively for a long time and it wasn't healthy.
Please, please tell her. I don't believe there is a single person here who wouldn't want to know if their partner was cheating on them.