Woah! People! The OP didn't know this guy was in a relationship - she's a victim of his philandering too. His poor partner. 
I would want to know if I were in her shoes.
I would say something like.
'Hi X. You don't know me, and I'm so sorry to be introduced to you under these circumstances. I am afraid that I am the bearer of very black news. I met your partner, X, on a night out recently. He told me that he was single, and I believed him. We ended up spending a lot of time together, and started a relationship. We had sex several times.
It was only recently I found out that he had lied, that he was actually attached and had a partner. Of course, I broke the relationship off immediately. I feel terrible - I am not the kind of person to have affairs with attached men! I am so angry with him for having deceived me this way, and deeply upset on your behalf as well.
I have been agonising over whether to contact you and tell you about this. In the end, I decided that I would want to know. I hope I have made the right decision.
I have screenshots and sexts to prove what I'm alleging here, if you feel that this would be helpful. I completely understand if you don't want to see or hear from me ever again - but I am also willing to meet up over a coffee if it would be in anyway useful or helpful.
I'm so sorry that this has happened. I am so sorry for the pain you must be going through. I would never, ever have become involved with him had I known. I realise that my pain in losing what I thought was a lovely, romantic relationship is nothing compared to yours at learning this about your partner.