I hate my best friends fiancé

I think my friend is in a toxic relationship and I just found out yesterday they're engaged. The guy has proven himself to be possessive, jealous, and overreacts when angry. My friend has children and has called her son a bastard (the kid is 10) and a few months back kicked them all out of the car in a remote place and abandoned them (there's a 3 year old too). I thought that would be it but within two weeks they were back together

I'm sad for the future of our friendship and afraid I won't be able to stay close to the kids