I think I hit the same problem occasionally, but I can usually keep trying different keywords until I find one that works. If he only tried 4 keywords like he writes, then there was probably some word in the email that would have been at least semi-unique. Maybe "earbud" or "order" or something. Then browsing or date-based filtering can usually do the rest. Kind of a pain, but not enough for me to ditch GMail completely.