Hmm, that's interesting. As the mom of a 16.5 yr old DS, I can't imagine him being interested in hanging out with a woman that age. He has a cousin that age, and they are on eachother's Instagrams and whatnot, but that's about the extent of it. (DS has more in common with her husband than her.)
At any rate, a few random thoughts.
I suppose it's a good sign she was so willing to come over tomorrow night.
She MUST know you're concerned and are inviting her over for that reason.
I think if it were me and I was sure about it, I would print up that statute that abdmom posted and give it to her as "food for thought" when we had a few minutes to talk alone.

I'm sure I would also tell her, in no uncertain terms, that I'd seriously hope that nothing inappropriate was going on between them. Yup, I'd just put it out there, woman to woman. That might be enough to make her re-think their relationship. (I think I'd also do more homework before meeting her. Is there someone you can consult to find out the legalities of it in your state, for sure? Police? Attorney?)
Something my pediatrician told me when we had a difficult situation with another person one time: remember that you have to worry about your son and she has to worry about herself. In other words, she is not your concern, your son is. I know you know that, but I thought I'd say it anyway because I found it helpful to keep in mind when things at times got a little confusing.
I'm not sure I subscribe to the philosophy of "leaving things alone because they're going to do it anyway". And I know for sure my DH doesn't, lol. Could it be possible your son is feeling a little overwhelmed with things and secretly desires for you to step in and "save him from himself"? IME kids don't always admit it in the heat of the moment, but sometimes upon reflection will say they were thankful for it, even if they may not have realized it themselves at the time. So maybe he wants you to step in...