The etiquette for correcting another person's grammar is that you don't, not unless you have blanket permission and a compelling reason to do so. Even then, never interrupt a train of thought or a serious conversation. The English Language has been around for 600 years in its present form, give or take a century, depending on which lingustic historian you ask, and is the dominant language worldwide for business, science and politics. it is, in short, sturdier than the average friendship and in need of less coddling.

Some people correct others' grammer more out of unthinking habbit than out of a deep protective instinct toward the mother toungue. It's a verbal tic with them, as swearing or automatically making wisecracks is for other people. As with these other peccadilloes, ignore it if it doesn't bother you, and if it does, gently register an objection.

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Irregardless
From Wikipedia, the free encyclopedia

Irregardless is a term that has caused controversy since it first appeared in the early twentieth century. It is generally listed in dictionaries as "nonstandard

Origin
The origin of irregardless is not known for certain, but the consensus among references is that it is a blend of irrespective and regardless, both of which are commonly accepted standard English words. By blending these words, an illogical word is created. "Since the prefix ir- means 'not' (as it does with irrespective), and the suffix -less means 'without,' irregardless is a double negative."[1]. (Cf. inflammable, flammable.) However, such double negatives are already found in the language in such words as debone and unravel.

Irregardless is primarily found in North America, most notably in Boston and surrounding areas, according to the Oxford English Dictionary, and was first acknowledged in 1912 by the Wentworth American Dialect Dictionary as originating from western Indiana. Barely a decade later, the usage dispute over irregardless was such that, in 1923, Literary Digest published an article titled "Is There Such a Word as Irregardless in the English Language?"[2]

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I think the use of the word is kind of like how some people here in New England will often be heard saying, "So don't I!" when referring to something they also do. They don't realize that what they said was that they do NOT do something, whereas they think they are saying they concur with you.