The OP stated this:
A small number stayed and slept at the apartment after the party I guess to sober up before driving home the next day. The girl and boy in this case were the only ones in the room where it happened and that morning he apologized for his behavior. The girl's memory is too foggy to accurately report anything.
However, here's the problem, what if he was apologizing not for sexual reasons, but because he made lewd or inappropriate comments? As the OP stated a small number stayed and slept at the apt. Assuming this boy lived at the apt., he could have excused himself, went to bed and had no clue that one of the small number of people staying were assaulting her.
His comment could have been innocent, but due to the lack of her memory she doesn't know if that happened.
She then fell asleep on the couch and in the middle of the night this same boy removed her clothes and fondled her and maybe more, but she doesn't remember much.
I have another problem with this story. She fell asleep on the sofa, and a small number stayed overnight. This implies it happened in front of people, because rarely do college kids rent apts/houses with spare bedrooms. Traditionally those that stay crash on the floor and the sofas in living areas, not bedrooms. Both of our kids have apts or houses off campus. I can tell you even 4 people staying overnight would equate to walking over bodies around the LR. Remember even a 15 X 15 room with a TV, sofa, 2 beat up chairs and a coffee table leaves little room for movement. The only way to get away with having sex in that small space would be both were quiet due to awareness of zonked out people in the room. Take a measurement of your family room, I bet it is bigger than your kids LR, now add in 4, 5, 6 people passed out...how much room is left? What would be the chances that a sober person would risk somebody waking up just to have sex in a non-private area? Now ask yourself, what are the chances of 2 kids drunk, would risk it if at least 1 during the heat of the moment believed it was consensual? Go back and remember yourself at 18 with hormones running amuck before you answer. I bet you will say a sober guy wouldn't do it, but a drunk couple would.
Not saying he didn't take advantage of her, I am saying it makes it more likely IMPO, that she was drunk and not drugged. Again, wrong to take advantage of her, but if he too was drunk, and she can't say he was able to determine that she was not consenting, because if she wasn't I am sure every woman on this forum will say, NO would be shouted, and physical harm would occur, which would awake those in the house from even a semi-drunk woman.
That also means he had to drug her and hope she stayed a sleep long enough for everybody to crash. Which now brings me back to why did her friends leave her behind? If she was crashed already when the friends left, than the friends would be able to say "when we left everybody was zonked". Not saying that a roofie wasn't given to her, I am just saying it is possible that she imbibed too much.
Many studies have proven that women who believed they were drugged and reported it have found that only 1-3% have actually been drugged, the 97% simply imbibed too much.