She can't leave her husband, she's married to him. She can't turn him off and on like a tap, he's a permanent fixture. So of course it's unreasonable for her daughter to insist she leave him or lose her.
Sadly, it's going to mean ' losing ' her daughter, at least for the foreseeable. I'm wondering if your friend is berating herself for being too lax but, in fact, her daughter may have been domineering (shock horror, some kids have behavioural problems which have no connection to the way they were parented). This crisis could have been waiting to happen somewhere asking the line.
If she's lucky she may have a decent adolescent behaviour team locally, at which she, and them all, may get the family input they need. Obvs the sf needs to tone things down a bit (all the very best when his twins get to teens) but it's all reached such a crisis they're going to need professional support and guidance.
I say if your friend is lucky bcs 1. The team may not exist locally or 2. they may exist but may be crap and make things worse she says bitterly. MH provision in this country is beyond crap these days.
She, they, may have to do what most of us have had to do: cobble what we can together and hope for the best.