Kelzie, I don't have teenage DCs, but I do remember being that age, and having parents who wouldn't let my boyfriend spend time alone in my bedroom, let alone stay over.
I had talk after talk about waiting, waiting for someone special, waiting until I was older, but it all fell on deaf ears. Consequently, we snuck around to have sex. In fields, in his car, in his friends' houses. I probably put myself at far greater risks by doing this, not only of being caught and arrested, but of contraception not working properly - a 15 year old using a condom for the first time in a field with long grass, desperately hoping to get it on right and also hoping that no one will catch them is really not going to do it as well as someone relaxing at home.
Yes in an ideal world, they would think of themselves as children with their lives ahead of them for sex and do nothing more than kiss and hold hands, but come on, that is so naive in the extreme that I'm slightly shocked anyone can actually post it in any seriousness.
But you are 'de-risking' the situation as much as possible. The only downside I can think of is that you wouldn't be able to say 'but I had no idea it was happening' if god forbid, she got pregnant. But you are doing everything possible to prevent that from happening, and I think your parenting should be applauded, not criticised, by your friend.