Move. Yesterday if possible.
This is NOT acceptable, on any level.
And I am saying this as a groom who has worked in show barns, as well as boarding barns. We NEVER used anything that belonged to a boarder, except on their horses. Not a lead rope, not a salt block, not a blanket, nor grooming tools. NEVER....EVER.
This is unacceptable, period. And I have to wonder how "big-wig" she is, if she is running a place like this. She sure isn't professional.
Most horse owners who board, will tell you the biggest deal breaker with them, is when their things are used on other horses, feeding is not followed, and horses are moved. And supplements, and special things, and meds are to only be used on the horse they have been bought for.
The owners, if it is a boarding barn, count on their horses being where they are supposed to be. We never moved stalls with Saddlers, and rarely any other barn either.
Although I do have to say, that where I am now, we move them around, as it cuts down on being so attached.
As for equipment? I would be coming unglued. If your things get damaged, or lost, who is going to replace them?
Find another place to board, unless you have the guts to stand up to her, but don't expect anything to change, as this has been going on for a long time I imagine.
Horses make me a better person.