The word at is a preposition, and as such should always introduce a prepositional phrase. Are you at home?
That being said, I want you to know that every English speaker breaks this rule in speech and in writing to one extent or another. They may not break this rule in a manner as egregious as the example you cited, but they break it nonetheless.
There is an old joke in American English where a poor kid gets to go to an exclusive university. On his first day he asks one of the upperclassmen, "Excuse me, can you tell me where the library is at?" To which the snobbish upperclassman replies, "This is Harvard (or the name of any prestige university) and at Harvard we don't end our sentences with prepositions." The new student replied "Oh, I'm sorry. Can you tell me where the library is at a**hole?"
By the way, you have friends who speak poor English not that speak poor english.