I tried looking for anything that would concur with his point, but all the sources Google Scholar gave me were very untrustworthy.

I did, however, find this neat little article on the "myth" of adrenal fatigue, from Crossfit.[0]

There's a part that says caffeine overuse can trigger general fatigue (or adrenal fatigue as refereed to the alternative medicine crowd).

http://library.crossfit.com/free/pdf/CFJ_2015_07_Adrenal_Exa...

((Of-topic, are there any other good research aggregators besides Google Scholar?))

Cochrane (http://www.cochrane.org/) is often a good aggregator for medicine-related subjects. Unfortunately, it doesn't look like they've got anything good for this one -- just a few things relating to caffeine and pain, pregnancy, etc.: http://uk.cochrane.org/news/caffeine-and-health-evidence-coc...

I like examine.com, they have a reasonable summary. It looks like there's not much strong evidence on any claim. The ones shown seem positive. https://examine.com/supplements/coffee/