Question Author
If they were having a proper relationship out in the open- going on dates etc... maybe it would be different as it would seem less like about sex.
It's not purely the age gap, it's more that she's so very young and he is not a good man. The last teenager he paid interest to was slightly older but had learning difficulties, a stop was put on that pretty quickly.
I was told he once made a sexual comment about his own daughter- even as sharing a joke with someone, if true, that is not appropriate.
Agreed that it is legal and none of my business, they are probably both pretty pleased with themselves, I just know that she'll regret it when she realises what sort of man he is.