Most Helpful Guy
If he was good friends with her before you guys started a relationship then you have no right to ask him not to see her. It would be difficult, I get that but 1 of the worst things people in relationships do is the "it's me or them, you need to choose" scenario. If she is a long term friend like a sister then you have nothing to worry about. Insecurities can ruin relationships so try to get passed it.
Most Helpful Girl
I think you're being insecure.
My SO has 2 best friends that are female. They grew up together and are really close. To begin with I felt a little threatened, but once I got to know the girls and understood the dynamic of their friendship, I was appeased. Now I'm friends with them too, and they're amazing people to me.
I think you need to get to know her better. Honestly, I'm amazed he's been so chill about your insecurities. If my SO told me not to see a male friend, I'd see major res flags.