I only read the title and first paragraph of this. That's all I needed to read because I just got out of literally the exact same relationship.

Tell me if this sounds familiar: she starts a huge fight over pretty much nothing. Goes where he is, or goes to hang out with him. Blames hanging out with him on you because she "just can't deal with you right now."

And everywhere she goes, he shows up, and SO says, "I had no idea!"

It is total bullshit and you know it is. Babe in the woods routine. You can feel it in your gut something isn't right about it. So does everyone else around you, including her parents.

Trust these things.

My ex and I have (well, had) a mutual friend that has ultimately escalated to what you're dealing with now. He started hanging out with her alone, more and more. Then sleepovers with no one else present. The emailing. The "good morning! how is your day?" gf/bf type emails. The picking fights with me over nothing and then running to her. Her showing up all the time, conveniently when I wasn't there, and him telling me "she randomly showed up!"

And the entire time, if I questioned it, or got upset, he called me crazy, insanely jealous, etc.

Then came the night I watched him lose it at the bar when she left with another guy. And I knew.

Honestly, OP, do these people think we're idiots?

It doesn't matter if nothing physical happened (and to be honest, with my ex and this girl, I don't think anything did.) But it doesn't matter. Emotional cheating decimates every relationship it happens in.

It's ultimatum time. Either she continues the "friendship" with him or she stays married to you. Doing both is now inappropriate given how you feel.

And no matter what she tells you, this is not your fault.

In my case? He didn't like the ultimatum, and broke up with me.

If you're lucky, she'll do the same for you.