S/he's wrong, simple as that. "The particular spatio-temporal version of speech that I grew up with is correct, and all others are bastardised" is not a defensible or - frankly - interesting position. Chaucer would find virtually all modern English to be debased; Bede would wince at Chaucer's English; and so on, forever.

Nothing fruitful comes from cultivating arrogance towards the language of others. It is just as much a cherished part of their cultural inheritance as yours is to you.