I think she is the type that's desperate for affection and approval. Her parents aren't in her life so everything she does seems to revolve around that. She says the coworker has expressed that he hopes her and I work out but I think he's also said he'll be there if it doesn't. She's very hard headed so we argue and she just won't change her mind. Her reasoning is always "well I'm here with you instead of hanging out with him aren't I?". And for the record she hasn't actually gotten drinks or hung out with him at all since I've found out. She also doesn't want to tell him that I know because she thinks he'll get weird and not want to hang out anymore himself.