all you can do is discuss with them why it's not worth it and be supportive and say things like i know you'll do the right thing eventually. in my experience if you just go at someone telling them how fucked up their relationship they get totally overwhelmed and shut down and feel shitty about themselves. other than that nudge them towards getting professional help because there's no way you can fix it. ESPECIALLY if it's your parent. you'd burn out before you ever got anywhere.

if you can't get them to a professional though you need to wash your hands of it or it will destroy you emotionally and you have to put your mental health first.