POD Deetee-----to me, the whole Connor issue is a red herring. If Jason (Jesse?) were my son, I'd never let him within a thousand miles of those adults.
Modeling healthy relationships is one of the most important things parents can do for their children. Connor's Dad obviously treats Mom like garbage, and she accepts it on some level. If Connor were a fabulous kid in every way, I still wouldn't want my son around that---I'd invite Connor over to play with Jason, if Jason was interested, but no way do I want Jason to watch a woman accept being treated like a second-class citizen.
What can the OP say? It's certainly going to be very touchy. I *might* try the truth, if I could do it in a gentle, non-preachy way..."Carol, when I saw Connor hit you and call you fat, well, I'm sorry, but I don't want Jason to think that's ok."
Carol isn't going to be happy about it, but what can she say? That is *is* ok to be punched and belittled?
Don't let her make Connor the issue (because, his behavior isn't about him, it's about what he's been taught, I believe). His giftedness is irrelevant, in my opinion.