Someone asks me to cite their paper. Should I cite?
I can think of a number of reasons to cite a source:

to acknowledge the work of others that you are building on (avoiding plagiarism)
to give credit to the author of a particular phrase that you quote (avoiding plagiarism)
to point readers to relevant previous literature
to signal that your work is at the cutting edge of the field, and
to situate your contribution in the context of related research
to provide evidence for a claim

&quot;Being asked&quot; is not among them.
Unless you retrospectively find that one of the reasons above applies (esp. 3-5), politely decline to cite the paper in question. In particular, you should refuse to add a citation as a favor.

Addendum: I just saw your update to the question, stating that your supervisor asks you to cite the apparently irrelevant paper. This complicates the issue. I would ask them to explain why and where to add the citation. Perhaps they want you to slightly revise your paper, incorporating information from the new source, which would justify the citation. If not, it depends on your relationship whether you are in a position to push back.