as a person who grew up with divorced parents, I know all too well about this
first of all I am very gratefull and glad that my father was apart of my life, even tho it was only every other weekend. My stepdad was a really mean guy, so every other weekend with my dad was my saving grace.
My dad DID have alot of girlfriends during the coarse of my childhood, and as a little girl I was always alittle jealous of them being with my Dad and taking my time from him. But I got over it, it didn't really affect me.
As long as she isn't doing anything bad to or around your children, I don't see a problem with it. He has every right to be apart of his children's lives, while having a life of his own as well