So this is pretty much it. My co-author, who is also the main author, has hired an assistant through Elance (an online work outsourcing platform) for doing statistical analysis and also to collect and annotate some relevant literature about connected topic X. I was aware of this and was OK with it, knowing that his or her work is not significant enough to claim authorship but will be acknowledged properly. Now it turned out that, for reasons not entirely clear to me, the RA does not want their name to be disclosed in the acknowledgement section. Even funnier, we do not even know their real name or email address, just the nickname and the profile they use on Elance.
We have the following options:
Do not acknowledge the contribution at all (at first, out of question; but see below why it should be considered)
acknowledge the contribution without naming the contributor. (Like "The authors would like to thank <Main Author>'s research assistant for their help in preparing this manuscript...") As far as I know this would be nonstandard.
acknowledge the contribution without naming the contributor but indicating that remaining anonymous was his explicit request.
acknowledge the contribution and identify them by their Elance nickname (Like "The authors would like to thank to "FyI1978" from Elance for their help in preparing this manuscript...") This would make us seem unprofessional or laughable; if I have the right impression.
redo the statistics and clean the manuscript from any elements that might bear the mark of the RA's contribution (so that we would not have to acknowledge them at all).
What is your advice, how should we proceed?