Are you sure they are aware of this? If yes, maybe they disagree but don’t have words or take action for some reason. Or maybe they know or think they know the man very well and trust him? Just hypothesis, i’m not really convinced.

But i’m even unable to conceive how someone that young would be attracted to someone that older, outside of any authority figure or idealisation kind of scheme. So I guess she must have a specific reason, and she’s doing this out of the blue, and this reason, whatever it is, may make it difficult for her parents to convince her or force her out of the relation.