Ummm she initiated a romantic relationship with him when he was 15, and had sex with him when he was 17.
She groomed him.
I don't know. I honestly get the impression that she's having gender issues of her own and manifest them on her son. I think she was looking for someone dumb enough to knock her up and pay child support. She just wanted a kid.
A romantic relationship with a 15 year old, may be sick, but not illegal.
If it turned sexual before he was 17, then there would be a problem.