My friend is part of a team of PhDs working on hard tech research problems without much interest in becoming a viable business (in the sense of counting dollars). It seems like YC would not be the right fit for them, is that correct? They seem to have gotten interest from other VCs but they have the impression that YC is more of a traditional firm focused on startups that are very close to revenue or are post revenue
Try and understand how you learn. A lot of intuition in technical fields, not just in mathematics, is understanding the material in a way that you understand it.
For me it was trying to frame as much as possible in a geometric lens. Draw pictures of anything and everything as much as possible. Graphs, number lines, 3d animations, whatever it takes.
Also, understand the "bold print" from the "fine print". I mean to say, every theorem will make a general statement, and then have a bunch of conditions where it holds. So worry first about the bold print, then try to understand the fine print.
Much of mathematics is about making the strongest and most general statements in bold print with as little fine print as possible. Basically the less fine print you have, the more important probably your result is.