Oh no....the 5D is worse IMO.....at least, I am compareing it without the vertical grip, which I noticed makes a difference.. It sort of "smooths" it.. I just learned this over the last weekend when the Canon rep was in town, and he displayed a 5D that had the vertical grip..( that has been on back-order for ever)... and I noticed that the mirror "slap" sound was lessened, and that it didn't feel as "shaky" when the shutter was released... I am guessing the same can be said with the 20D..(I think)..
your buddy may need to get a better tripod, head, or mounting plate, or all three.. When I need to shoot long exposures, I always use CS function number 12, which enables the mirror lock up mode.. I'm not sure why that wasn't detailed before,... Also,....I am not sure which lenses he is using.. If he is using slow lenses,...and long exposures,...I could see him getting shake problems..more so than fast lenses because of the bulk of the lens.. it actually adds to the stability of the camera, given that everything else is rock solid...(tripod, head, mounting plate..)....Oh, on that note, I prefer a non cork mounting plate.. I actually remove the cork, and have metal on metal,...which lessens the vibration,.....this is most helpful with longer lenses.. That, and, I guess this would be a possibility,...that if he is using a lens that can be used with a tripod colar,...that he should use that thing, rather than having his camera mounted by it's tripod mount on the bottom, rather than the tripod mount on the colar of the lens...I have seen plenty of "misuse" like that believe it or not..
For me,....I haven't had any issues with mirror slap being too bad.. under 125th on a lens that is â‰ˆ the same focal length.....I'm sure you know the rule I am referring to..
Ask him for more basic info,...as like in... what lenses, tripod,..mounts..etc.. he is using..