Legally the case is clear, but that aside, I think, based on the one issue, pedophile seems too harsh. I have three reasons for this;
One, we don't know the maturity of both involved. We know their age, sure, but as someone said on page one, there can be huge differences in that age range. That's biological fact.
Two, when I was a teen (90s Europe) plenty of my friends had older boyfriends. My first serious crush had one. She loved me too, but wouldn't separate from him. Many of them would drive Vespas or cars. It really was a rather common sight. Even the parents didn't mind. This was before mobile phones, so an older teen driving and protecting their daughter was maybe seen as preferable to hitchhiking, which was a serious danger. Plus, I too had my first experiences with an older woman in her 20s. I wasn't groomed, abused, nor do I regret it now, because it's the only normal sexual relationship I had before my mind went to shit.
This is important, because, three, the girl from back then, now a woman, still maintains that she didn't feel taken advantage of, just as I. One should respect that. It's one thing making up rules for society as a whole, that's fine, but telling her or me what to think is patronising, especially now as grown ups. This consent is crucial, because it's what separates normal sexual desire from abuse. It's the difference between pleasure and trauma. Sex and rape.
No, to me the real question is what attracted him to the girl at the time and that is indeed where my alarm bells went off too. Why is he still so concerned with this (supposedly) onetime issue after so many years and her expressing her good will? Even going as far as wanting to ctb because of it and now therapy too. That sounds extremely shady and suggests there's more to this. Without all the details one can't say, but where there's smoke there's fire...
Personally, I believe you that you are speaking for a friend, but of course, if it weren't so, this would literally change everything.
PS: Totally adored my video rentals dealer, must've been 15, she was around 20 and if she would've had me... sigh. She even asked me one time, if I wanted us to get together and I was so embarassed that I blushed and denied it. She was messing with me. Haha. Loved her so much.