But is it right?
When I want to know where a movie is playing, I don't call a friend. I call Moviephone. When I want to know what year World War II started, I don't hit the malls, clipboard in hand, to survey all the shoppers. I open a book. When I want to know whether monkeys qualify as intelligent life forms, I don't ask the woman who does my hair. I just turn on C-SPAN and watch our elected representatives in action.
But when it comes to grammar and usage, some people seem to think that anyone who can answer their question can answer it well. I think this stems from a nearly universal perception that, in grammar, every question has an irrefutably correct answer. So if someone says, "each other" is only for relationships between two, no doubt he learned that from a teacher or from a book and therefore it must be the correct answer. Right?
I've said it before and I'll say it again. In grammar and usage, often there is no correct answer. There are only varying degrees of consensus as to whether an answer is a good one. And for "each other" versus "one another," I'd say our questioner was given a pretty good guideline. But that's not the same as a right answer.
"Garner's Modern American Usage" says, "Usage authorities have traditionally suggested that 'each other' should refer to two people or entities, ('John and Bob helped each other'), 'one another' to more than two, ('All of them loved one another')…. Careful writers will doubtless continue to observe this distinction."
Sounds pretty solid. It is pretty solid. But now hear what "Fowler's Modern English Usage" has to say.