MIL giving toxic advice

My SIL just turned 20 and is in an extremely abusive relationship. Her fiancé continues to beat on her. My MIL is best friend's with her fiancé mom and it was honestly then trying so hard to put them together. Now they are together and he BEATS her. My MIL has been telling her that he loves her and she should try to change him. That is terrible advice to give your own daughter! You can't change an abuser. They can only change for themselves. So MIL keeps telling her that she can change him. She will just need to be the perfect wife to prove herself. So now my SIL won't listen to what I tell her. She just says she will marry him and she will prove her love to him so they can be happy! My husband always said his mom was an extremely selfish person, but this is awful. Just because she's best friends with her fiancé's mom. My SIL is also so young so of course she's taking her moms word for it. I'm just so pissed my MIL is doing this!