I pronounce “merry”, “Mary”, and “marry” as homophones, as many Americans do, but a good many other Americans (and, I believe, a higher proportion of Brits) pronounce all three differently.
I referred him to vol. 1 of Accents of English, which he seems delighted with.
It’s not just that “a higher proportion” of Brits distinguish the three sets. As far as I know, all do. To the best of my knowledge no native speakers of English outside north America lack the three-way distinction merry — Mary — marry (RP ˈmeri, ˈmeəri, ˈmæri). We do not rhyme sharing with herring. We do not rhyme clarity with prosperity.
Just as this fact may come as a surprise to Americans, and seem problematic and mysterious, so it can be a surprise for non-Americans to find that some Americans make no distinction. And Americans can therefore get confused over spelling in cases where we never would.
Words like merry belong under DRESS, those like marry under TRAP, and those like Mary under SQUARE, which historically is derived from FACE. So you can often tell which word belongs where by reference to the spelling. Before double rr, the spelling e indicates DRESS and a indicates TRAP. With a before single r the vowel may be SQUARE, as in vary, parent, aware, compare, garish, Carey; but because our spelling system does not consistently distinguish short and long vowels before a single consonant letter it may also indicate TRAP, as in arid, apparent, comparison, circularity, Gareth, Gary. The suffix -ary is a special case.
The DRESS-TRAP distinction, as we know, can be a trap in EFL. A Danish friend of mine, now dead, was telling me about an acquaintance whose name I heard as Berry. An unusual name, I thought, but not impossible at least as a surname. It was only years later that I discovered he was really Barry.
The day before yesterday the Sunday Times travel section had an airline cabin attendant recounting how
a very anxious-looking couple boarded a Chicago–Heathrow flight. … They were studying the route maps intensely and staring wildly around the cabin. I asked if everything was all right, and the gentleman said, in a broad Midwestern American accent, ‘Are we all going to perish?’ I thought ‘Oh dear, here we go’, and assured him that we were not, but he just became even more upset, pointing at himself and his wife, then saying ‘We’re going to perish.’ I put my hand on his shoulder and told him in my most soothing voice that there was no way that they or anyone else on the plane was going to perish, but this had the reverse effect. It was only when my supervisor came over that we realised that they were going to Paris, and hadn’t realised they had to change planes at Heathrow.