This sounds like the cyber version of being accused of rape. Simply ask “you are making a serious charge, do you have any evidence; as I consider this slander”. Reverse the aggression, bullies don’t respect weakness, and this woman sounds like a seriously disturbed bully.
Who goes on Facebook, asks to be a friend with a person they suspect molested their sister? Does that sound like a rational person?
So, chose to drop that person from your account, I believe the term is “Ignore”; then if you get a message - take the aggressive approach. Accusations of Slander, extortion and harassment are applicable in this situation, IMHO.
If he did nothing, he’s being forced into a sitution that is literally impossible to prove. How does one prove their innocence? Simply stated, you can not do this.
What you can do, is state that nothing happened .... PERIOD. Accusations to the contrary are harassment, slander and can easily lead to extortion. Tell her that she is not to communicat with him at any time henceforth, and that such an attempt will be met with a restraining order, and legal action against her.
If nothing else, it will make him feel better; knowing that he is not helpless in this matter. He does have alternatives to simply ignoring her, and feeling shamed into admitting something that did not happen.
Very good advice.