If you and he didn't have any kind of agreement to be monogamous in this "situationship" then he's guilty of lying to you but not "cheating" on you. Whether he's "cheating" on this other woman depends entirely on the nature of their relationship. If they're not exclusive then him being with you isn't "cheating". But obviously if he's promised monogamy to her then he is "cheating" in also sleeping with you. This guy sounds like a pig, sorry. But no, don't tell his girlfriend. Just refuse to see him or talk to him anymore. In all likelihood she won't believe you anyway and then you've made this (seemingly duplicitous) man into an enemy. The potential for him doing some revenge seeking on you is too high to even bother with this. She'll figure it out eventually because people like this guy don't tend to change.