The easiest thing to do is leave. I don't know anything about your relationship, but...unless you can somehow cut that toxic metamour out of your life, I'd move on.
I stayed with a partner with an abusive partner. But I made it clear, once I realized what was happening, that a condition of my staying was them keeping that person out of my life. Actually, I never even had to do that, he told me himself that he wouldn't let that toxic person around me.
I hate ultimatums, but I think, for my own sanity, I would have had to leave him, even then, if he had not decided to leave her.
I know it sucks, but please know that you can't help a partner who refuses to help themselves.