A few months ago, a PhD student I had never met sent me an email asking me for advice about their paper. I have written papers on related topics and apparently I answered a question of theirs on the relevant SE site.
The paper was interesting and I was happy to give advice. However I found a mistake in a crucial lemma that puts several results in jeopardy. I wrote back with the advice and added an explanation about the error. I never received a response (not even thanks for the advice), and the preprint remains on the public preprint server untouched.
What should I do at this point? Do I write back asking for an update? Do I wait until the preprint is updated?
I guess what I am afraid of is that the paper contains results I am not unlikely to use in the future. But if the proof is wrong, that's a problem. I would like to avoid the public humiliation for the student to write in one of my papers that their proof is wrong and that I have a correct proof, or even worse for the paper in question to be published (because the student didn't care that I found an error) and then corrected/retracted once I inform the editor of the error. On the other hand I may be overthinking it as I am not responsible at all for these things: I don't know the student, they asked me for advice.