Question - Did the sub-panel ( Remote Distribution Center ) have a Sub-Main Breaker or a Main Lug Only panel?
Your question did not state if the "sub" had it's own breaker or not.....so if it did not and their was an overload on the entire panel and possibly causing issues with the feeders to that panel...it may trip the breaker in the Main Distribution Panel......it's possible.
Here is what I would do......
1.) Check the connections to that 100A breaker in the main panel, ensure they are properly tightened and all connections are sound.
2.) View any of those same conductors for overheating…just to be safe.
3.) Does your friend have a Amp-Probe Meter…he could check the amp draw on the feeders to this panel…and see what he had…
Also could simply be a breaker going bad…what is the age of the setup?
It is important to note that knowing exactly if something is overloaded in a panel takes a calculation…however, doing this 20 years all I would have to do it see what is connected in BOTH panels and give you a good guess.
Now…what is important is…does the “sub” have it’s own main breaker for that panel…or is it a MLO panel?
Check all connections…check for discolored connections, terminals or discolored insualtion at the termination points as well.