Though (I suppose) this thread is closed, may I add that, for me, "someone else's" is quite clearly a reference in the "possessive" sense, rather than "plural" sense.
On the other hand and as has been already discussed, "passer-by" and "passers-by" (hyphens may be incorrect as has been pointed out) are references with respect to numbers, being singular and plural.
If a speaker/writer wanted to make a plural reference still maintaining the "possessive" sense that is meant by "someone else's", definitely the word "someone" will have to change to some other word, maybe not even one single word as I can think of. Like "Some other people's" or something else.