Not really. Plagiarism means something else, and padding your paper or citing some random guy doesn't help you in any way. (A lot of work is done to shorten papers to get them under journals' page limit, and many have at least a full page of citations.) And he only did it once; if nothing else, he can plausibly say "I had these papers on my desk, had a deadline, and wanted to make sure I wasn't accidentally using their work without giving credit".

[I'm doing a PhD in crypto after my MSc in math, my girlfriend is doing her PhD in math, lots of friends are mathematicians.]