As long as she continues to work across the hall from OM, she will get her emotional needs met by him. It would be extremely hard for her to be strong enough to drop him. I'm not saying it couldn't happen, but things are likely to turn to a PA before she's through with him. Do you know what you would do if you found out she had slept with OM? Would that be a deal breaker for you?
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!