If it was brought up in court there would be questions about her age, how threatened/unable to say no he felt etc etc.
The fact that a court might have questions about it doesn't mean she didn't humiliate, violate and upset him to the extent that it affects him to this day.
It bothers me that the question seems to be around some sort of law-based definition. Whether the law sees it as a crime doesn't much help the victim in terms of how they feel about it (though of course a conviction can help, by creating a sense of justice).
Think of it this way OP. If your friend had a friend who badgered him for a long time about handing over his bike to the point that he handed it over against his wishes and feels upset about it to this day, would you be asking 'did the friend steal the bike?' It'd be a pretty silly question wouldn't it? Regardless of whether it's technically stealing, it's a mean, aggressive act that upset your friend.
Why is it with sexual assault it always comes down to definitions and the law? Stupid question of course, I know the answer - because creating doubt around sexual crime is rapists' main defence and one of the ways they do it is by insisting everything is framed in legal terms - it confuses people, makes them believe they can interrogate victims and question their feelings, when they wouldn't dream of doing the same thing in other similar situations.
Don't get sucked in.
Your friend was badly hurt by someone. That someone was horrible and manipulative. Your friend deserves sympathy and support, not a discussion about definitions and comparisons.