What warrants acknowledgement or citation in a dissertation?
Citing in this case is unnecessary. An acknowledgement would be gracious, but also not strictly necessary. My personal inclination would be to do it if their explanation led to some insight on my part.
But &quot;Thanks to JDoe for their insights into Theorem 35 of Gauss&quot;, or whatever, is, as I say, gracious.
It is even less necessary if the person's job is to teach you stuff.