Is this the same guy that spends so much time online?
I think the 18 year old girl is looking for a sugar daddy, and wants to be a mom, but needs practice with other children first. If this 35 year old man can't leave the kids out of it, he's pretty pathetic. Especially having these babies in the car when she's still learning how to drive. So is this what he's teaching his grand children? That if you're a man, you can have affairs, and keep a sugar mama and a mistress? And if you're a woman, it's okay to have your heart broken by a man who cheats?
It's too bad that a teen crush can have such an impact... It's actually quite normal, say in a college setting that an 18-20 year old student has a crush on their 30-something professor, but usually it just stays that, a crush.
I think your son in law is going through a mid life crisis that is not even in existence, and is too young for. Someone needs a backbone.