I don't plan to support my girlfriend's friendship with another man.
Do you think that she's involved with him?
I don't know, but I think she's flirting with him.
That is your girlfriend's business. You should butt out. It's her friend.
Eh I just feel so insecure and like I'm going to lose her. I want to call some of her female friends to prove it.
It's not unusual to feel insecure when your partner has a friendship with someone of the opposite sex. It's important that you talk to your partner about your insecurities, otherwise they'll only grow. You can't tell your girlfriend not to maintain a friendship with someone, she's her own person with her own life.