How 'around' is around? I mean, the odd social engagement on a monthly basis isn't too bad if they're in the same social circle. I probably wouldn't appreciate it if they're on the phone everyday but again, a call every now and then or an email is fairly normal. You just have to decide if he is treating her the same way he treats other friends or if there is something specific in the way he treats her that is bugging you.
Ultimately, there's not much you can do, I certainly wouldn't go down an ultimatum route if that's what you're thinking but being open about it with your boyfriend is fair enough. And remember, they did split up so it obviously wasn't going far.