> Generation V introduced a mechanism often referred to as a "Shiny lock", preventing wild Reshiram, Zekrom, and Victini from being Shiny. Any Pokémon obtained in Entree Forest (i.e., from the Pokémon Dream World) are also prevented from being Shiny. Reshiram, Zekrom, and Victini still have Shiny sprites in the games' data.
Also, Smogon doesn't allow you to use a shiny Victini in a teambuilder because it can't be caught shiny. :P
Bulbapedia also says so. Many youtube videos are fakes and could have used RNG manipulation, hacks, or bad emulation to make Victini shiny. :P
Smogon and Bulbapedia usually get information like this from data miners- literally the exact way the game was programmed, and in it Victini is Shiny locked in said game code. You friends could have lied to you about finding Shiny Victini without hacking, or could be using a fake copy of it without realizing. Did you see their shiny Victini? :P
Also, shiny Victini cannot be transferred through Pokémon Bank and Pokémon Bank only keeps out hacked/impossible Pokémon (except for shiny Jirachi prior to a previous programming oversight in other games being alleviated with Shiny Jirachi's release). :P
I trust Pokémon Bank and the Gamefreak Game Developers themselves more than you and your friends, to be frank. Sorry if that sounds harsh. If you had a legitimate source, I may be enticed to look into it. :P