Do I tell a girl I've never met that her boyfriend has been cheating on her?

I've known this guy for a few years and we've always got along well but I always knew that he had some kind of feelings for me (as he had told me several times). Last summer he told me that him and his girlfriend had split up. We started talking more and around October time we started seeing each other. I started to get stronger feelings for him, we got along very well and he was on of the very few people I felt truly comfortable with. One day he had told me he was going to be sitting down with his ex to talk things over which made me a little uncomfortable at first but they had been together a few years so I let it slide. By February/March we had been seeing each other quite often for around 5 months when he went started going quiet. A couple of weeks passed when I saw on social media a picture of him and his 'ex' holding a set of keys, captioned "we got the flat". At this point I didn't know how I felt. After looking into his ex's social media it was clear to me that they had never broken up in the first place and he had been seeing me behind her back. I felt dirty, hurt, used and guilty. One thing that really got to me was the way he used to speak about me to me, claiming I was an incredible and beautiful person inside and out and he really loved spending time with me. I can't help but feel terrible for his girlfriend who appears to be completely oblivious to all of this, and for all I know I'm not the only one he's been seeing behind her back. I feel like out of respect I need to let her know what's happened... not because I want to be with him or want them to split up because I don't want anything more to do with him and want him out of my life completely. But I feel if I was in her situation then I would want to know even though it would be heartbreaking. They've been together years, and I don't know what to do because it's been playing on my mind ever since I found out they hadn't broken up and I don't know how she'd react if I were to tell her the truth. I have many screenshots of texts from him that show we were seeing each other for quite a while and he had lied to me about them splitting. The dates on these also match quite a few on her social media posts relating to how much she loves him etc. He has yet me once or twice recently too. I just don't know what the best thing is to do but I can't get it out of my head!

The best thing to do is to walk away with your head up. Let his GF find out just what sort of a person he is as she will in time. There's nothing to be gained by keeping texts as proof that he was two timing and it doesn't matter how his GF would react regardless of where and who the proof came from. Ask yourself if you would feel any better if you did show her texts because you then, would become further mired in more difficult circumstances where the healing process for you would be delayed. It's YOU who has to heal from this situation and the best way to begin is to say nothing and cut him out of your life completely and move on.

I wouldn't get involved. I have a similar story. I had this guy beg to date me a few years ago. We went out a couple of times. He kept on telling me he was ready for marriage. I didn't really care for him and felt something wasn't quite right. It turns out he was ready for marriage, as he got married to his 5 year fiancee about two weeks after we dated. I found out because his wife wrote a lovely testimonial about their devotion to each other on their wedding site. You don't know what the girlfriend sees in this man. If he was dating you he is probably dating other women too. She'll either figure it out or has such supremely low self esteem she doesn't care. Or maybe she likes the drama. There will be an explosion for these two at some point, don't let them drag you into their issues.