Wooden structure came with the property upon transfer of ownership---unless the old owners are going to move the trees and all
The portable one is a little more dicey. First, your friend should check with the old landowners---that' s probably the easiest and safest. If they put it up (or more than likely at least know who did), then the proper owners can be notified.
As for retrieval, in CT & NY, both of which I' m familar, you legally have 30 days to remove your belongings after moving. The key word is moving, not closing. So if the property closed on August 1, but the old owners did not move (for whatever reasons) until September 1, they have until October 1 to remove all of their belongings.
After the 30 day period, legally the stand would be considered abandoned and becomes your property.
I' m not a lawyer, but I' ve moved enough that I looked into the legality of the property laws.
Hope this helps.
S&