Email evidence, proof enough?

My friend’s been dating a guy who she finally got intimate with him after one year. So now she feels confident enough to go through his emails in front of him. He said “Go for it. I’ve got nothing to hide.” She then finds an email from craigslist where he’s going back and forth with a guy saying that he’s gay curious. She confronts him and says he’s just playing head games with a guy who originally emailed him about his couch for sale. She lets it go until she finds another email thanking him for a donation to the Gay Men’s Health Crisis. So let me ask you. Is this enough evidence to prove that she’s dating a gay man? Not that there’s anything wrong with that. Is there?