The odds are - just based on the long history of rape and sexual assault - that she is telling the truth. Sure, sometimes rape accusations are made up. But that is a small minority of cases.
But I am not comfortable with the notion that behavior as a kid - a drunk kid - is indicative of who this man is now. I am also not comfortable with the notion that a he-said/she-said situation from 35+ years ago can disqualify a nominee. If this was just one mistake by a kid, then it is different from a pattern of abuse.
So far, it's just this one mistake by a kid, maybe egged on by drunk idiot friends. He's not Trump, who groped and raped his way to the White House.