Idk, I think women are kind when it comes to sex. I'd rate myself a 5 as a lover but never had a woman tell me to do a better job (even though I'm sure they all thought it at some stage, lol). Ok, a 6.
Point being that was never the focus of arguments or dissatisfaction in my relationships...as far as I know!
We're lacking data here. If he was with them for a few weeks or months then maybe it's the sex. Years - it's stuff that can't get improved. Only men are convinced that a good dicking will cure all problems.