I haven't played it yet, so I don't know if its true myself.
This is FALSE! For starters, Victini is shiny locked, so it can not be shiny unless given shiny through an event. Second, no trainer in game has a Victini, not even a regular one. Third, online services such as GTS and battling in White 2 (and all Gen. V games ) have been discontinued. So, sorry. Your friend was lying.