Well, it depends on the context. I think it's probably best not to lecture or scold anyone about language use if it doesn't otherwise appear that they are being racist. And probably not a good idea to scold anyone about racist/sexist/classist this, that, or the other. Not many of us enjoy being scolded and its probably not especially effective in getting the point across. So, unless there's a good reason to be hateful, don't.
On another list, a guy just (very) mildly scolded people about using the term Native American instead of American Indians or Indians (for short). Much to his chagrin, he learned that one posters husband is a lawyer who specializes in American Indian issues. As she said, "Regarding Native American/Indian, my DH, who works with Indians on a daily basis, says most of them prefer Indian, and use it themselves. "Native American" is what people from Washington - who mean well but don't know anything - say." The other guy's avocation is studying American Indian culture and he agreed, though he said he also knew a few folks who preferred NAtive American.
Moral of the story: best not to be hateful.
The person who used the phrase may not have a clue. Until I googled it, I had no idea that it is a phrase uttered by American Indians in B Westerns but not a phrase found among any AI tribes. Not a big Western movie fan. IF, as Michael said, he was saying it without the "a" then I would say the person may be mocking language use. But again, is it really useful to be hateful about it? What do you get out of it if you are hateful about it? (Not that I think you would actually be so -- I suspect you'd be a lot more cautious and caring than your words, above, indicate.)
kelley
"We live under the Confederacy. We're a podunk bunch of swaggering pious hicks."
--Bruce Sterling