I feel like you kinda started in the middle of the story. So you are taking your son and his girlfriend to something the following day that requires leaving at 7 AM?
I think there are two things you have to consider. The first is that your son and his girlfriend fool around. I don’t think you need to worry about this too much, honestly. They’re not sleeping in the same room, or even on the same floor. And assuming they ever go anywhere alone together (like to the movies or something) they could be theoretically be fooling around there. (Not trying to put that image in your head, just saying.) So I don’t see how this is worse just because it would be in the middle of the night. I highly doubt he’s got the cajones to try something, but you know him and I don’t so…
The second is how it looks, aka, “scandal.” Is it going to get around school that his girlfriend slept over at his house, and all the other kids are going to make assumptions that something sexual went on? Is that rumor then going to float back to the parents, and now there are all kinds of rumors swirling around? This is the more serious concern, I think.
I guess bottom line is I think you can probably get away with it, provided you are crystal clear on the rules, and her parents are on board too and giving her the same speech.