Hi Scrant, I was pretty blunt with my H and told him I wasn't willing for us to spend time as a family whilst he was in a R wih someone else. I'm a stepmum, rather than a Mum though. I know some others on the forum have a more flexible view on this in the interests of the kids. But, TBH if your S is 15 and doesn't want that anyway, I would avoid the contact. I think your W is trying to cake eat and is in denial about the consequences of her actions, and it sounds as though you're going along with that from fear of losing her.
I think 'brief and friendly' is okay - but do you want to hug and kiss someone else's girlfriend??
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus