Am I a red flag? And advice on how to stop being one?

So I've been dating my girlfriend for a year now, and she has a guy friend who asked her out two years ago but she rejected him but they still remains close friends. I'm fine when they hangout in friend groups, but they often hangout alone and she described him as someone she would vent to and can emotionally depend on. When they do hangout alone I find myself being uncomfortable even though I trust my girlfriend 100% . So I made a dumb move of talking about a friend in a similar situation and she called him a red flag and an insecure guy. I'm not sure how she would take it if I let her know directly

Now am I a red flag?

If yes then how do I change myself?

how do I handle this situation?

TL;DR: I'm uncomfortable with my girlfriend hanging out with a guy friend that asked her out