It's not your business. Let them handle it. They're reaping what they've sowed over 15 yrs of parenting and there's little they can do now.
Raising kids is like paving a road. When they hit their teens, they pretty much have to drive on the course you've laid out. But, you get over a decade to do it, so there's no excuse for it leading off of a cliff. You can finetune their travel on that road in their teen years, but you're not going to change direction seriously.
If the girl's above the legal age in the state, you should try to get yourself a piece too...