i rmb when i was p3/4, i did go to my friends' house. and quite frequent type. of cos, beforehand i will ask my parents and give them my friend's house no so they can call up to 'spotcheck' haha.
i think it is okay to let your girl go, but ask for their house no and ask if the parents will be at hm or only the kids. at times, we parents need to let go abit =)