Gilrfriends friend is ratchet what do I Do?

My girlfriends best friend is reckless a poor decision maker and polar opposite of my gf. I've suggested to my gf that we all hang out but her friend denied two opportunities to hang out with us.. I assume this is the 3rd wheel problem but how can I fix this. I don't really know her friend except from what I've heard from my gf and very little of what I've seen. Most of it being negative and just a bad influence. I explained to my gf that it's hard to be comfortable with her hanging out with her friend but my gf says she's "getting Better"... I don't understand how she expects me to just be okay with it when they go out.
Gilrfriends friend is ratchet what do I Do?
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