YABU. Let her go and make sure she has her mobile with her.
I would be more worried about the girls having a falling out than anything else, that is much more likely especially if there is a group of them!
Send her with a bag of sweeties, some magazines plenty of nail polish and makeup for makeovers and she will be the most popular girl there.
It is hard enough to make friends at that age, facilitate the friendships she does forge where you can.
Of course you should go in and meet the parents first. I would personally probably ring them in the week to discuss the arrangements so you know what's what.
The worst thing teenage DSD did was say she was having a sleepover at friend X's house and it turned out that they stayed with the friends older teenage sister instead. No-one was very happy when that was discovered so I would probably check the arrangements with the parents each time a sleepover is organised.
Plus I think your no term-time sleepover rule is mean too, obviously it is reasonable to have no sleepovers during the week but at weekends I don't see the problem.
My mum hardly let me go to sleepovers when I was at secondary school and it was really alienating and my friends stopped inviting me to lots of things and I am still pissed off about it today!