Four Letter Word for: What is a Jointly Held Work?

A 91-year-old visitor to the Neues Museum, Hannelore K., filled in an empty crossword that was part of an artwork by Arthur Köpcke. According to her lawyer, Ms. K. was following the instructions “insert words” and is claiming she holds copyright as a collaborator over the “augmented” work. Ms. K. is threatening to sue the museum claiming when the museum repaired the work, it violated her copyright.

This brings up a number of questions, such as,  could everyone in trouble for copyright infringement or damaging a work make this claim? Could the ability to claim this nullify many copyright and VARA protections afforded to artists?