How can someone be a co-author in a paper by just presenting undocumented evidence?
I think the true answer depends on what the other co-authors think.  In my field and particularly for application-oriented papers and case studies, we often add industry representatives or clinical staffs as co-authors, even when they don't contribute much to the writing of the paper.  

We do this because it fosters good relationships, demonstrates their facilitation of the analytical work, and (in our belief) is good a "people-practice." 

For others, we simply list them in an 'acknowledgements' section with some comments.  

It all depends.