I'm going to hang out with a girl who stole from me and my best friend.
Why would you hang out with this girl?
She never paid me my money back so I want to find out why.
You should talk to your friend about the money you lent her and find out when she will pay it back. It's likely that she just needs more time to give it back to you and she may be too embarrassed to bring it up.
She didn't pay me back because she didn't know me well enough to feel bad. That's why I will hang out with her until she realizes her errors.
Do you know this for sure? I still think it'd be wise to go to your friend about this first.