My friend purchased the "Plan: Staff of Herding" from the potion vendor in Bastion's Keep, but she doesn't remember where or when. I know the Plan, like all Whimsyshire items, is account-specific, so I'm not asking about the Auction House—are there any other vendors who sell this Plan, or is it just this one potion vendor? When is the plan available for purchase (between which quests), or is its appearance in the merchant's store randomized?
Yes. In Bastion's Keep Stronghold the merchant "Gorell the Quartermaster" will sell the Plans to create the staff during the final quest in Act 4 "Prime Evil". He is right beside the healer NPC.
As pointed out by Blem, this only works for the improved version of the staff. The normal plans have to be dropped from Izual.
No, Plan: Staff of Herding only drops from Izual, he can drop them on any difficulty. The vendor Klokworkk is talking about is only for higher levels of the staff - see this.