If you set the boundary, you need to stick to it if you want her to respect you.

She can choose to talk to OM, but the consequence is that you may choose to leave the M if she crosses your boundary.

You haven't told her she cannot talk to OM. She's a big girl, and can make her own choices about who she talks to or associates with.

She can make whatever selfish and foolhardy choices she wants, but also needs to be able to feel the consequences of those choices.