Cautionary tale on asserting what is good or bad English

Two weeks ago, the anguished letter below was posted in the Personal Messages box of Jose Carillo’s English Forum. I am presenting it here in full with very minor editing, but I have withheld the name of the letter-writer and of the English teacher involved to protect their privacy. It has taken me this long to act on the letter because I wanted to avoid judging or moralizing on a matter that’s not entirely the province of English grammar and usage, but in the end, I thought it would be for the public good to just share the letter as a cautionary tale about the perils of asserting what’s good or bad English.

Here’s the letter: