Does the person who downloaded the data deserve to be a co-author?
The proper answer here is clearly that this person does not deserve to be an author.  Downloading a bunch of data files is not any kind of intellectual contribution to the research.  It may have been a necessary task, which was required to get the project operational, but the same could be said about administering your institution's e-mail server or keeping the HVAC system you use operational.  Workers that do not contribute intellectually to a project do not merit authorship on papers, although they could be thanked in the acknowledgements, if they put in a large amount of time in support of the project.

You seem to be concerned that your funding agency may not want to see people paid to work on a project who are not authors on the resulting publications.  While some agencies may have concerns about this, what they are ultimately worried about is whether their money is being spent on the research it was intended to fund; they do not want the funds redirected to other projects.  They must also understand that some money is going to go to people who do not end up making intellectual contributions to any papers.  Some people just end up doing scut work; some people quit before they make any progress; some research projects unfortunately never produce anything publishable.  All these situations will be familiar to the funders; they may not be thrilled about any of them, but they know that they do happen.