She's a person, not a toy. She's in it now. She can choose to leave if you tell her you feel like she's coming between you and him. You can choose to stay and get over jealousy. You can choose to leave. He can choose to leave her to preserve your relationship. But you can't make choices for them amd if he doesn't want to dump her and she isn't willing to leave there isn't anything you can do about it.
Sue, hinge in a vee with Nate (Polysexual) and Sam (monogamous)