It's interesting to see this topic. I've been in this situation (as the boyfriend). I'm glad you are looking to take some action because I know it's not fun (for anyone involved) when your girlfriend feels insecure about friendships. As James points out, the actions you take might send a message to your boyfriend that you don't trust him and that can actually destroy the relationship which is what you are trying to avoid in the first place. I'm highly aware of this whenever my girlfriend has seemed insecure and we talk through it. I hope your boyfriend is aware too.
Is your boyfriend clear with them that he is just interested in friendship? I try to make this clear when a girl is coming onto me. It usually goes something like, "I really like you, but I have a girlfriend, so I'd like to explore being good friends with you, but nothing romantic." Some girls only want a ♥♥♥♥♥ and I rarely see them again, but I've made some good friends too.
I hope I can learn from this topic too. This is still an occasional issue in my life and it helps to see it from another girl's perspective. I want my partner to feel secure and I also want freedom to have female friends.