<human>: Background: Started casually dating/sleeping with this guy about  3 months ago. We hit it off and had crazy chemistry but neither of us wanted anything serious for various reasons. We agreed not to sleep with other people (at his request) and have been hooking up around twice a week ever since. Half the time I would sleep over (again, at his request). He always seemed genuine and I thought he was one of the good guys. Even though it wasn't a real relationship, it was sweet and tender and he always said stuff about how he cared about me and thinks women should be treated with respect and blah blah blah...stuff that is probably bullshit knowing what I know now. Which brings me to... 

The problem: Through some internet searching I found some pretty substantial evidence that he has a serious girlfriend of 4+ years back in his home state. I know he isn't in an open relationship. There is a small chance they recently broke up but she doesn't use her social media accounts anymore, but that seems like a stretch. He previously told me he "hasn't really" dated anyone in the last 2 years, so I know that at least that much is a lie. 

The question: Do I confront him about it? Do I tell the girlfriend? I should note that I am 100% not jealous and I fully accepted the non-serious nature of our relationship...so that isn't a factor here. Really just feeling stupid for being duped and feeling bad for the girlfriend.

Can you give me the gist of the text in a nutshell?
<bot>: Found out i've been the other woman for 3 months. Do I confront the cheater and do I tell the girlfriend?