'He don't' is slang or the incorrect use a verb with a personal pronoun. Your choice. ;)

Yes, that is grammatically incorrect, but Americans will say that. I cannot explain to you why people say it that way, but it sometimes relies on region and education. Not always. Some people just think it's cool to be grammatically incorrect. The proper phrase would be "he doesn't" or "she doesn't." I've found the easiest way to figure out if you're using a contract properly is by reading it in your head with the lengthened version of the contraction. This also helps when you're trying to figure out the difference between "there," "their," and "they're," as well as "your" and "you're." EX: He don't = he do not (WRONG) He doesn't = he does not (CORRECT) My trick for "there," "their," and "they're" is to always remember this phrase, "They're there with their friends." "They're" meaning they are, "there" referencing a location, and "their" being the plural possessive form. (not a part of your question, but just another even thing Americans get confused on).

Those are primarily part of the Southern United States and inner city vernaculars, or dialects. I'm not sure how commonly these are used outside the U.S.

i noticed that too