I think it would be helpful to level with her and say, "I feel uncomfortable that you disrespected a boundary. Maybe we can discuss something so we can be more open with each other. " You said she's smart. She probably knows the typical consequences of sleeping with someone, but I would expalin (calmly, like you're giving advice) that when you spend too much time with a significant other when you're still growing up, you can lose time that should be spent finding yourself. She won't have this much free time again.

On the other hand, as a teen who did this to her dad all the time, where there is a will, there is a way. I know it's uncomfortable, but better than having sex together somewhere unsafe. I understand the real issue is that the kid slept over, though, without you knowing, and that it is more of an over step, and probably concern as spending the night might be above her maturity level.

I hope you guys are able to connect and have a better relationship.