I can almost guarantee you she will look back at this with remorse, regret, etc. given what you've described. Some people, especially young people, use kink as a form of self-harm when they're struggling with something. I wish I didn't speak from personal experience.

Also, your husband is treating this young lady like a sex object he can toss around, injure for pleasure, and leaves the emotional labor to you to boot. Please take a good, hard look at who he is as a person, because what people do in the bedroom is not separate from the rest of them. This is not the behavior of a kind man who respects women.

Thank you for caring about her enough to question this and ask for assistance. She might be staying because she's lonely, hurting, struggling with something, for your presence and care, and so on. Either way, I wouldn't allow this dynamic to continue.