Teens can be complicated. I know. I have 5 kids. Youngest two are 14 and 12.
How close is your relationship to your daughter? If you are close, you can probably do nothing other than point out how weird things are, such as they way they won't discuss their beliefs, or the slavery thing. She'll probably eventually exit the situation on her own.
If you aren't close or you are prone to flying off the handle, that can be used as a wedge between you and your daughter.
Either way, this only ends in a few ways:
- Your daughter indoctrinated into a cult with you cut out of her life and labelled a "subversive person" (unless you decide to join, too)
- Them splitting
- Him leaving and losing all of his family, connections, and support
Scientology is a cult. And not just a cult, it is a vicious cult who take revenge on anyone they perceive as an enemy. People have found themselves surveilled and having their bosses called making things up to try to get them fired. They are extremely litigious.
Obviously I wouldn't want my kids in any cult, but Scientology would probably be the worst one. At least most other cults, if you make yourself available to help when they decide they need to leave, they can escape. Scientologists will literally house them on a prison ship.
Good luck, friend.