Having had my formative years in New Zealand, I was born in South Africa. I vaguely recall when I was VERY young having someone tell me when I said "hey" that "hay is what horses eat".
I got that then it was more when people would yell "hey" across a room to attract attention, and that was considered a bit rude for that purpose.
However in my adult years in NZ and the UK, a casual nod and a "hey", or "hey, what's up?" was a perfectly acceptable greeting.
Having recently moved to Canada, I greeted someone casually with that yesterday to be told again "that's what horses eat". I felt a bit rebuffed, but decided to ignore it until I could research it - ie, ask on here!.
Is this just a North American thing? Or just possibly this individual's view? Same age and gender as me.