Your daughter is brave to share her story. And her honesty is a part of recovery. The volatile relationship is hindering her recovery, yes. And all you can do is pray, love, and support. Two people close to me were in similar relationships. They were as addicted to their partners as they were to drugs.
They got as much dopamine from the horrors of their relationships--chasing the highs and plummeting in the lows--as they did from drugs.
Recovery is about recovering your life--your whole life and your whole self. No one can give it to you.
Again, thank your daughter for sharing her story and her honesty may very well help someone else. I hope it helps her too.