Nope. Your problem isn't the OM. The problem is that you are showing big time weakness...
Your answer is to NOT GO OUT WITH HER WHILE SHE HAS ANOTHER MAN.... PERIOD...
YOU be the one to walk away from this.
Women respect a man more when he shows emotional strength by doing things like telling her he isn't interested in seeing or dating or being with a woman in a committed relationship.
You wait it out until she dumps him and chases YOU. If not, so be it.. Let her see you are a happy mature man going somewhere with his life. Keep dating other women and don't be afraid to let her know it...
She can't respect you if you don't respect yourself enough to not get into a relationship with a woman who can't pick one man and stay with him. She can NOT be trusted and you deserve better. The only way to show her and you that is to live it.