Should I add a co-author?
As you wrote,


  Sarah's contribution to the paper would be small, and perhaps not normally worthy of a co-authors credit, but without it the paper would be incomplete.


The recommendation is to add her to the authors of the paper. I think that little things would be the gem of your work versus the similar works and you don't neglect the small valuable information with her experiences through working with the technology. 

Moreover, adding Sarah as a co-author doesn't reduce the impact of the paper, but it also promotes the validity of your work.