What should I do?

You take it seriously yourself, and you get her counseling. Start there.

You may need to go nuclear, as in, yeah, a legal intervention (don't start with this, though). But start with counseling. Good on you that you found the problem. But you can't do this alone. It is above your pay grade.

Arrange for counselling for her.

It would have best if you'd started it when you first found out about the abuse, but now, do it now.

What sort of abuse? We throw the word around pretty casually these days, so unless it's actual criminal physical or sexual abuse, and not just normal childhood bullying sorta stuff action may not be required since it's usually a modern adult's interpretation of something screwed up by the modern PC nonsense.

In general spanking is tried and true. Shouldn't be used to inflict pain but serve as an attention-getter. "You've done something I told you not to do, so to help you remember here's an attention-getter."

In the US, we've second-guessed ourselves from what's been proven to work to correct undesireable behaviours with modern crud of time outs and other misapplied psychological concepts. Yes, negatuive reinforcement works less well than positive but usually because it isn't carried out properly. As with the perverbial "Wait until your father gets home, then you're gonna get it." For negative reinfor cement like spanks to be effective they must be done right away so the child associates the negative behaviour with the punishment.

No one ever became a psychopath or serial murderer because they were spanked. But many serial murderers had no discipline at all growing up.