Originally Posted by response from MW
Dear Mr. Hanratty:
As explained in the following article from Merriam-Webster's Dictionary of English Usage, the use of "on line"
instead of "in line" seems to have originated in New York City, but now appears to be spreading. (When I was growing up on eastern Long Island in the late 1960's through early 1980's, "on line" was already quite well entrenched there, so I would imagine that it has probably extended itself even farther by now.)
So who's right? As dissatisfying as my answer may seem, I'm afraid that you both are. The simple fact is that no two English speakers use the language in exactly the same way, so there are always going to be regional--and individual--variations in pronunciation, meaning, usage, etc. Some English speakers differentiate between the first vowel sound in "Mary," "marry," and "merry"; others do not. Some speakers call a sweet, carbonated beverage a "soda," while others call it a "pop" or a "tonic." None of these variations is more valid than the others; they're simply used by different people living in different places. My advice, then, is to bury the hatchet over this one--or at least avoid lines for the immediately foreseeable future!
I hope these comments are helpful.
Sincerely,
Karen Wilkinson
Associate Editor
Merriam-Webster, Inc.