I found this article about correcting other's grammar.
These are the two viewpoints on why you should/shouldn't correct someone's grammar.
Barry Leiba, a technical researcher at IBM and author of the delightful blog "Staring At Empty Pages," said there are "exactly four" situations where it's all right to correct someone's grammar: (1) when you're an English teacher correcting a student, (2) when you're coaching a nonnative speaker who's asked for help, (3) when someone else has asked for coaching, or (4) when someone puts the equivalent of a "kick me" sign on her back.I agree with the person who says you should only correct someone's grammar in narrowly defined situations. Obviously you understood what the person was trying to communicate if you can correct them. There are many other (and less embarrassing) ways to advance someone else's best interest. Why decide to have someone else's best-interests in mind while only correcting their grammar?As business guru Alan Weiss, Ph.D., explained, when we let people get away with grammar goofs, we don't have their long-term interests in mind.
This is called operating in the other person's self-interest, Weiss said. It's not an easy thing to do when you're correcting the grammar of someone who can fire you. But, as Weiss put it, "ethics are not situational."
"It's like being partially illegal or partially pregnant," he said. "There isn't any such thing as partially ethical."