My baby doesn't trust me

Is this still a problem for you two? Ill answer anyway, I've been through this and when I would see him flirting with another girl no matter what he said the relationship was. It'd hurt. A lot. And I would just say screw it. I'm going to flirt too.

is she really bad with flirting with other guys, or is that just her being herself?

plus, that's how relationships start. As friendships, then as close friends. And so on.
=/
hope this helps

Can you approach your friend in a less flirty way? You CAN change your behavior if you care to. It's not beyond your control... It's about being considerate. If you really want to make things work with your girlfriend you can talk to your friend and let her know you guys need to tone it down so you don't appear so flirty. If she's your friend she'll understand and be supportive of your relationship.

Of course your girlfriend sounds like she has issues with trust among whatever other problems you guys have. Is it worth the fight?