Probably isin't maybe it just feels that way?
Regardless, dude got screwed and I wish him the best.
It's actually the other way around.
In a case of pure he-said/she-said, far more often than not, the man accused of rape is not convicted. There needs to be corroborating evidence, which may include but is not limited to: an immediate police report, forensic data collected from the victim immediately to prove they had sex, pictures of bruising or other marks that indicate a struggle, eye witness accounts, admissions of guilt by the accused.
I'm sorry for what happened to your buddy, if indeed he is innocent as he tells you, but I'd have to see a lot more of what went on with that case before I believe he got convicted wrongfully.
In this particular case, I can see two complicating factors that may have had an impact on Banks' lawyer telling him that a plea deal was his best bet. The first complicating factor is it sounds to me like he admitted they had sex. Without knowing a LOT more details, it's entirely conceivable that there may not have been physical evidence of the deed itself. That may or may not be true, I odn't know all the details of this case...just saying, if he admitted to having sex with her, that presents a problem.
The second complicating factor is the fact that she was 15 and he was 16. It is a little known fact but even if you're underage yourself, it's illegal to have sex with a minor. This particular age differential, her being 15 and him 16, may present a problem in that depending on the state's laws he may have been at the age of consent whereas she was still technically under it.
Whatever the case may be, it SOUNDS to me like whoever his lawyer was (court-assigned public defender?) at the time he was 16 and being accused...just completely screwed him over and gave him bad advice. I would say that happens a LOT more often than guys being thrown in jail for rape because it came down to he-said/she-said, without any marks or bruising to corroborate her story, and him being sent to prison because the courts sided with her story. Truthfully, I would say that almost never happens. But getting bad advice from a court-assigned public defender who has no interest in doing much work on your case and probably thinks you're guilty anyway due to his/her own prejudices...that happens.