See one reason (and just one) why people don't report it. Going to his flat - at 16, no less - is
evidence of complicity. The person above is unfortunately not unusual in this line of thinking.
Then they've got to deal with the police, many of whom have shown themselves to be misogynist in their "banter" at best, rapists and even murderers at worst. Who'd trust them with stuff like this? I wouldn't. Then if they relive their ordeal enough and manage to get the CPS to allow it to get to court, they have to wait at least two years where they can be violated again, and for what?
A tiny chance of a conviction which even then might well not lead to any real protection for the victim from their attacker.