OK, I'm in Star's camp here, I'm probably about to offend a whole lot of people.
First, thirty-somethings on the prowl. I don't dignify them with the word "cougar" which, frankly, is rapidly going to become an offensive term in its own right.
What the H is a 30-something doing on the prowl? Especially when she's married? I will tell you - because she is already a repeat failure when it comes to relationships. She can't maintain a relationship once the passion passes. A younger male has passion. And don't be too sure it won't last - because the age difference MAY work in this direction.
As for stability - I apologist in advance to all the sane psychologists out there (both of you) but in my experience, the most unstable, paranoid, difficult people I have ever met have been psychologists. I'm not speaking professionally here, they seem to be able to hold it together in a consult. But when dealing with them in their personal lives - ratbags, the lot of them (except for you two).
I'd check the label on the bottle of lotion she was using on him - see if it says anything about "good for improving coat condition" on the label. Because to her, he's just another pet, a stray who followed her home and stayed because she fed him from her plate. She collects interesting things, cute fluffy wind-up toys that make her feel like a mother figure in control. The sex is a fringe benefit.
A friend of mine was similar - she was a teacher who had an affair with a 17 year old male student, while still married. When she threw her husband out he would cry on our shoulders about her but he would still go and babysit his kids for free so she could work without having to pay for childcare. He paid support for his kids, but she also drew a single parents pension with child care allowance (supposed to subsidise the cost of childcare). She got the house, she got the car - he had to ride a bicycle to work and to stay with the kids. Meanwhile her boyfriend finished school and moved in with her when he turned 18. I believe they (she and the schoolboy) are married now, 20 years later. They've had two more kids. I've met him a couple of times years ago but she and I only exchange letters these days.
So let him have his fling. Her reaction to you - she is jealous. You're his mother, and SHE wants to be. At 30, she's sublimating her biological clock by having sex with the child she wishes she had had.
Sick.
I'd be taking a leaf out of Star's book and refer to your son in canine terms around her. "Make sure you check that hairy little crittur for fleas. He had mange when he was in Kindergarten."
Marg