As to why he would defend that, it's just simple relationship bias. We'll hesitate a hell of a lot more if it's someone we like or think we know.
You should have seen the irrational tenacity with which one of my parents stuck by a pedophile, right up until their first few nights in prison, simply because they were dating. Even if it meant losing everyone around her. She was the only person under god who couldn't see this a mile away. His own kid was his first accuser, but surely not, right?
What kind of thing could your best friends do that you wouldn't initially defend them over?
These two have known each other for literal decades. I'm not too surprised he would refuse to accept it from someone he's been friends with for that long.
I am shocked that he would choose to step down entirely over it. I could see the question presenting itself to him, since it's...not wonderful PR...but I would have thought the cause itself would have been more important to him. He's between a rock and a hard place.