Scientific authorship: including a former lab member who collected (unused) pilot data?
There are two key questions, as I see it.  The first is whether the experimental design you used was originated by the former student.  If it was the PI's idea (or somebody else's), and the student merely implemented it, that the former student does not need to be an author.  If, on the other hand, the apparatus and techniques were truly originated by the former student, there may be a strong case for making them an author.  You should ask the PI specifically about who designed the experiment (as well as consulting them about the whether the previous student should be a coauthor; the PI is likely the best person to evaluate whether the student made a meaningful contribution to the current paper).

If it is established that the old student did originate the experimental arrangement, there is then a second question:  Was this experimental design already used as the basis for a published paper with that student as an author?  If there was already a paper on the older results, then the student has already received credit for designing the experimental setup.  In that case, you can cite the older paper as originating the methodology, and the former student does not need to be an author on papers describing further developments made after they left.