My
local Walmart and Home Depot carries them now. I think it is an idea that is catching on.
The biggest issue is a lot of people cannot care for their own pets, much less a tree that looks well off until it reaches the critical point. That is, a live tree is like sheep, by the time you note it is under stress and unhealthy, it is too late to resuscitate it. So they let it go unwatered all winter until the ground thaws out and they can plant the thing, by then it is either dead, or dies from the shock of being replanted.
Still your idea is great. I have planted many trees and a few special ones for those who have departed.
Please accept my sympathies for the loss of your friend. I know loss is a part of life, but to grieve is normal too. I hope your tree grows for as many years as you anticipate in their honor.