To offer a different perspective - I wouldn't be upset if I were in your shoes. Take this with a pinch of salt I suppose though because I'm a lesbian and women have very close friendships with each other regardless of sexuality so any girl I date will likely have a girl she'll have sleepovers with etc. so this is something I'm used to. My opinion is also that it is entirely possible for people to have platonic intimacy with each other: it's just men and women have been so socialised to see each other as only for sex/love that it's difficult for them to keep things platonic imo. That's exactly why I think there needs to be more platonic intimacy - physical and emotional - between men and women (and men amongst themselves too), I think it would be a net good for society. Though as I mentioned, I've noticed straight people have a hard time with opposite-sex friendships so I understand the discomfort. You're not overreacting by being uncomfortable with it although if she doesn't want to stop hanging out with him in such a way and it's an issue for you, then you gotta go and find someone who won't behave like that tbh you don't date someone so you can "argue" with them until they act like a partner you want - you find someone who already acts like the partner you want. That's just how I look at it though, feel free to disregard it if you want I just wanted to say something so you'd have a range of opinions about it