Therapy and honestly put your foot down on her not seeing him anymore, because the risks are too big. Even if she hate a you for a while rn. Explain to her the possible risks explicitly as reasoning for why you can't budge on this topic (physical abuse, sexual assault, him putting her in dangerous situations). See if you can monitor the messages he sends. If he treated her in any way and you have proof, you can get a restraining order.

Both of you read Why does he do that by Lundy Bancroft, a few chapters a night. Start with the chapter that explains signs of an abusive man, and then talk about it togother every night.

Because ago I raised her well up till now she should be more open to listening than you think she would be.