As Coffe and Tea pointed out, none of us can say with 100% certainty what we would have done in that situation when we hear secondhand information.
A graduate assistant comes and tells you he saw your best friend sexually assaulting a 10-year-old boy. I imagine the reaction to this would be first shock, then doubt and denial. Then you wonder how you should respond. Go to the police? Confront your friend? Report him to your boss?
Let's keep in mind that once a person is accused of these things, even if exonerated it never, NEVER goes away. Maybe JoePa thought the best course was to keep it within the university until evidence came forward? Maybe he thought there must have been a misunderstanding? I don't know.
Lots of blame to go around. At the university and law enforcement, who apparently took their own sweet time investigating and obtaining an indictment.