I guess I am afraid if I push to hard she will leave.
She does constantly remind me that she's not leaving her house.
It's just that OM seems to badger her so much that she starts thinking about it. Then when he's gone for a few days she changes her mind and stays.
a) If you are basing your actions on your fear of her leaving, you will never succeed at this. Work instead, always, from a standpoint of "what is the RIGHT thing to do in this situation?"
b) Based on your comments above, you know very, very little about setting and enforcing boundaries. They are NOT about controlling, and in fact if done properly they totally LIBERATE the other person to do whatever it is they want to do (so long as they are not hurting YOU).
If I could recommend two books to you, they'd be "Boundaries" by Townsend & Townsend, and "Love and Respect" by Eggerichs and Eggerichs.