Tell your husband you want your best (male) friend sleeping over. How will he react?
I'm sorry this is upsetting you and for the situation. When I met my husband, he had female friends. Once married, really, I'm his significant other and the female friend thing was much less. In fact, any girls that didn't try to befriend me and just wanted to be HIS friend and not OUR friend really got the cold shoulder and he isn't friends with them anymore. NOT because I told him to do this but because he felt this was best. He saw our relationship over any friendship and even if I wasn't best friends with a girl he was friends with, the ones that stuck around were always nice to me and supportive of our relationship and respectful that *I* was the wife.
So, what about this. I agree it would hurt my feelings if my husband put any friend over me. Guy or girl. I want him to have friends and support his friendships. So, I'm not trying to have him all to myself. But if it is a guy friend that acts like I don't exist and he wants my husband alone, always, without me. I'd feel strange and my husband would probably pull away from that guy. I pretty much always have an invitation to hang out too if I want to but I usually don't. ha. Let him have his guy time.
How does this girl treat you? Is she friendly? Are you friendly to her? Before ending your marriage, what about trying to befriend her. This would make your husband happy if he is truly friends with her. And if she tries to rub the friendship (her and your husband) in your face with inside jokes and such, just point it out to your husband later and ask him what the deal is. Kill the situation with kindness and see where you get. If she isn't receptive to friendship---- let your husband know with examples. If he sides with her, I agree that is problematic. good luck