It's likely he has and/or is abusing others. If the victim you know won't come forward, then you'll have to look for other victims who will be willing to come forward. There's no way a person can be put away if no victim is willing to testify, because then there's no case.
Frankly, I hope she does come forward, for the sake of others who may get abused in the future, especially given that her mother has clearly been completely supportive. It's obviously up to her though and nobody should judge her for not coming forward. The case may not even go to trial (guilty plea) if it's open and shut, and once she comes forward the prosecutors and police will start looking for other possible victims.
The Constitution give the defendant the right to be confronted by his accuser.
It's certainly understandable that your friends sister doesn't want the humiliation. Perhaps your friend can help support her in what I consider a moral act of courage.