You are talking a lot of s.o.p.s here.
Along time ago, one of the rear screwholes was "tinned" to give a 0v or neutral point. The negative voltage was negative with reference to ground, the positive was positive with respect to ground. You could have 24 volts between the 2 or 12 volts to one or the other.
Some bright boy noticed that the tinned holes crushed less often than the non-tinned holes, so people started tinning all the holes to make thier product more solid.
Cheap mobo makers put traces too close to some of the holes, so a large head screw could damage them. Another bright boy decided that he would invent fibre washers to save those boards.
As for me, I never use the fibre washers, too lazy, but the excuse I use is that the tinned points do a good job of conducting heat from the board to the case, which, to a small degree, is quite true. Every little bit helps.
By the way, does your friend use a fibre washer on bothe sides of the board? If not, he isn't isolating anything, and is wasting his time. (unless of course, he is using crappy mobos and large screws.)