My understanding is that Lisa Jeter, close friend of Sherri's for whom Sherri used to babysit, brought Cameron Gamble into the case.
In this interview with Jeter and Gamble on Dec 1st, they credit their Facebook efforts with attracting the anonymous donor. Watch short interview here:
http://m.onenewspage.com.au/video/2...-Gamble-Speaks-About-Sherri-Papini-Return.htm
During the 20/20 episode last night, Keith Papini said that he has absolutely no idea as to the identity of the donor.
It is then explained by the narrator that the anonymous donor contacted Keith (a letter appears quickly on the screen) and directed Keith to get in touch with Cameron Gamble. That bit starts at the 17 minute mark in video below.
[video=youtube;h22BCV7bPcQ]https://www.youtube.com/watch?v=h22BCV7bPcQ[/video]