Well done on speaking out!
It might not be possible to explain, I'm afraid. This happened when the Jimmy Savile stuff started coming out - or, rather, when he was dead and it started being discussed. You know how people idolise their idols (!) It's a bit like telling a firm believer that the water at Lourdes is actually a bit crap and they've been mugged off for the money.
Depending on how you value this friendship, it might be wisest to go for an "agree to disagree" on this. Hopefully the stories will be proved (I believe them) and of course it's personally symbolic to you.
Some people just can't take the nastier facts of life. I stand shoulder-to-shoulder with everyone who's been forced to look those facts in the eye ... but I've long since given up trying to reveal them to those whose lives have been more protected!
As you know, speaking out is a positive step. For you, for those victims, and for the many who have yet to speak. It's up to whether you continue to be friends with those who can't "hear" you.