I don't know why r/r your brother-in-law's RD took so long. The most time-consuming part of that is securing the chain so it doesn't flop around and scratch stuff, and even that only takes a few minutes, some foam rubber and some tape or string. Couple of turns of a 5 mm Allen wrench either way, and I've never needed to so much as re-adjust the cable.

I do it for added security regardless. Even though my bike is small enough that my long cage RD will fit, it's just another thing sticking out to potentially transmit an impact to the chainstays.