I believe that the rumour about Fernando being the source of some of the leaks started from that interview he did with a Brazilian fan site in 2001, where he said that he was letting his friends listen to the songs:

Fernando: I've listened to the new record, and I can say that all the songs are beautiful, not even one track is filler. The record may have 16 or 17 songs and they're all very strong. I don't say this because I work for him, or because I'm his friend. My friends, for example, whenever they listen to any of the songs, they always say it’s very good. "Catcher in the Rye," "That's Called the Blues," "Chinese Democracy," they are all very beautiful.
https://www.a-4-d.com/t4766-2001-07-dd- … ndo-lebeis

I haven't come across any further hints that support this rumour either in the 105-page pdf or elsewhere though.

I've also seen speculation that the leaks of later demos with Bumblefoot (Silkworms, OMG, Going Down) were sourced from Fernando's friends who he shared the files with, but I don't know about any evidence for that either.