Should I tell her?

So this guy I work with told me that he has feelings for a girl we work with. He says that he can't stop his emotions towards her. He has a girlfriend who he lives with and they have been together for 2 years. He said that he would never leave his current girlfriend but that he hates that these emotions are there. I've never talked to his girlfriend, I just know of her. This guy is also friends with my SO. Should I tell his girlfriend or is that over stepping my boundaries? I would want someone to tell me. I'm not one to step into someone else's business but i feel like this would kill me if it were me.