Now that is the way to remain friends with her. I would do whatever it took to remain neighborly friends and it sounds as though you are doing the right thing. Especially if you are not near the property all the time. She could be that extra "eye" that is sometimes needed on vacant land.

I would just add something to the letter about "what if" someone attends the shed and gets hurt, OR "what if" something happens to the shed in light of it being on your property.