Is it acceptable for a boyfriend to be best friends with a girl who he has hooked up with in the past?

The 'best friend' is a female, calls him all the time when the girlfriend is around, she hates the girlfriend and has admitted that she likes the guy as more than her friend. They have also hooked up in the past. But he says there's nothing going on and he's not attracted to her at all, but he likes hanging out with her.