Having been a whistleblower myself (in a completely different scenario involving someone who volunteers with children) I can very much understand why the whistleblower did not speak up immediately.
When the person being criticised is popular, experienced and applauded and their behaviour is implicitly condoned by everyone around them then it is very psychologically difficult both to believe your own instincts when they seem to differ from everyone else's and then to speak out.
It may well have taken a while for the whistleblower to process what they witnessed and to work out in their mind that this behaviour was definitely not acceptable. It may have been hard to trust their own instincts when those around them who should be expected to know what is ok and what is not seemed to think the behaviour was acceptable.
It may have been a difficult psychological process to accept that their own instincts were right and the many people around them who seemed to think it was acceptable were wrong. This would presumably be even harder when the whistleblower was a child and the adults around them seemed to be condoning the behaviour - maybe even their own parents.
Even when the whistleblower accepted the behaviour was definitely wrong, it may be that there were other people around who did agree with them but made the whistleblower feel that they could not speak up because no one else would be willing to back them up and no one would believe them.
Again from my own experiences I know that there are only a very few people who are actually willing to put their own head above the parapet to publicly criticise poor behaviour. Many people are willing to privately criticise but then go quiet when it comes to putting themselves out there to stand by their criticisms.
Any whistleblower has to be extremely strong to stand up and speak out and face the criticisms they will receive and to stand by their accusations when others don't publicly back them up.
In addition many people's instincts seem to be to feel sorry for the person being criticised - see some posts on this thread. It is really sad how many people focus on the feelings of the abusers and not on the fact that they abused the vulnerable and that it is the vulnerable victim who should be focused on.
It is the person's own unacceptable behaviour that is the cause of the criticism and if they didn't want to be publicly criticised then they should not have behaved in that way.