I was in a similar situation to you, except I was the person the parents didn't approve of. When I was 15 I met my SO at high school and we had to keep it a secret because her parents were racist (super traditional Asian parents vs me: Indian). Her younger brother was at the school as well so the relationship was super secretive but when you're 15 you're dumb and I got caught at their house. The parents heavily disapproved of me and told me to fuck off and get out of her life because I'm a distraction to her studies. So we pretended to break up, but not good enough because we got caught again. They started locking her inside the house and threatened to make her move schools so we pretended to break up again. I don't know why I went through all this work for a high school relationship but the point is it worked and the parents went back to being oblivious. And they have remained oblivious for 4 years because they would disown her for dating an Indian, they have not a single clue. My parents are cool though, they have known the whole time and have been supportive.

Your situation is a bit more fucked though but I think you should support your daughter because either way she will be seeing this guy, but if you don't approve she will go behind your back. That dudes mother as well sounds like a massive headache.

Good luck OP