you put yourself in the position. Honestly I wouldnt board my horse with a family member or close friend. It usually ends up messy and hard to talk about without creating a rift in the relationship.
I DID board with an extremely good friend for awhile, but we both knew it was a business arrangement, and acted accordingly.
Neither of us ever took advantage of the other under the guise that we were friends and it would be 'okay'. Not everyone can do that, but we did.
OP, I'm sorry you can't have the horse. Unfortunately, when you're at the mercy of someone else's whims, you have to take the bad with the good.