He's a cheat, should I tell her?

I met someone about a month ago, we got together on a night out, he wanted me to go back to his that night but I didn't. For about 3 weeks after that we were talking every day, he'd sent me nudes within about 2 days and was talking about how he wanted to book a hotel room for us, the things he'd do to me and there was a couple more times he wanted me to go back to his after a night out. He was meant to be coming to mine one night I had a free house but completely stopped replying to all my messages. I found the instagram he'd told me he didn't have after realising a lot of what he'd said didn't add up and he's got a girls initials in his bio. A bit more investigating and I found them both on Facebook and it looks like they've been together for over a year. I want to tell her how unfaithful he's being but also don't know if I shouldn't be getting involved in other people's business. If I was in her position I'd want to know but I know some people would think I was sticking my nose in. Should I tell her? Please help I'm losing my mind!

***I will probably tell her but it would be nice to hear other people's opinions and what they would do***