You wanted a lecture, here you go:
You can use less for countable nouns, any of them. We've been doing it for literally centuries. In fact, it has never been used only for uncountable nouns (unlike fewer, which has generally only been used for countable nouns). Correct language is determined by what native speakers use on purpose, not what a textbook or teacher says.
At least read the Wikipedia and the dictionary if you want to keep a strong opinion about this:
However, modern linguistics has shown that idiomatic past and current usage consists of the word less with both countable nouns and uncountable nouns so that the traditional rule for the use of the word fewer stands, but not the traditional rule for the use of the word less. As Merriam-Webster's Dictionary of English Usage explains, "Less refers to quantity or amount among things that are measured and to number among things that are counted.”