My girlfriend doesn't like my best friend?

I have a friend who I have some light history with. We used to have feelings for each other, but the title has always been friends. Eventually, feelings faded, but we’ve been close ever since, and hearing about each other’s new love interests have never been a problem. However, she huge flirt. Fast forward to now, I got a girlfriend. My girlfriend doesn’t like her because this friend is still really flirty with me. I don’t entertain it, and I’ve talked to my friend about how and why I feel guilty whenever she pulls stuff like that. I have no feelings for this friend anymore whatsoever, but I don’t want to lose her because we’re so close. What can I do to allay my girlfriend’s concerns?