

You are not wrong, but age gap should be considered relative to the people age. 20 years difference between 20 and 40? I guess, still not the best thing ever… Between 17 and 38? It’s very very different, people mature A LOT between 17 and 20. Then of course it depends on the person, maybe this 17 years old is mature for her age, but maybe she is just being taken advantage of.
Also, the wording of the mother really implies it’s an assimetric relationship, not really one between consenting adults, of course we don’t know, but still…
That is true, for an adult issue, a serious and adult conversation must happen with this girl, even if it could be okay, the situation itself should be evaluated because it can get very gnarly. We don’t know much about OPs way to handle it, but it’s also possible this specific situation is fine.
Either way, I agree that just telling your daughter to stop doing something she clearly feels like doing, even if she has a distorted view of that something, is not going to help anyone.