How do we get our daughter to see she is making a huge mistake with the guy she's going with? It has nothing to do with social status or anything like that. We have just become aware of how he has treated some other women.
You need to have concrete information. If you don't, she will probably go in the exact opposite direction you think is good. Even if you need to hire a PI, don't go to your daughter without being absolutely sure about what you want to say.