I've read a ton of stories like this on Reddit and I've had it happen to me. He will never set boundaries with her because the "work wife" fuels his ego. Sometimes she is actively interested in fucking up another woman's relationship for funsies, sometimes she buys into the bullshit of their "special connection" that could never be unethical or weird because they just understand each other sooooo well. He'll either get bored with her eventually or actually cheat on you. I wouldn't wait around to find out what happens if I were you. You don't have to put up with this crap, feeling your stomach tense up every time his phone goes off and wondering what is going on.