My girlfriend asked me to hang out with her ex, should I let her go?

My ex just asked me today if she could hang out with her ex who’s also a female I don’t know her name or age and she wants to hang out today and they say they might be indulging in a bit of a paraphernalia. I don’t really want to let her go, but she also don’t have any female friends. The reason I am opposed to it, not out of insecurities, but what if it brings up a Situationship between them and out of all people her ex I don’t mind my girl having female friends or hanging out with anyone as long as not someone that she used to have relationship with. Do I sound delusional
My girlfriend asked me to hang out with her ex, should I let her go?
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