Southern Ontario rural-ism, meaning, she is dumber than a dog... The longer version of the phrase, which is seldom spoken, is "She don't know sick'em from fetch!"
Did you hear your Aunt Sue, arguing around the table with everyone?! There is something WRONG with that woman! She don't know sick'em!
When your problems,especially your love-life, get too much for you and you, unexpectedly, 'throw-up' all the stuff going on in your life and through your mind over a friend (a third party). Neither of you expected it to happen and at least you feel better for it afterwards though your friend often wonders what they had done! Probably made the mistake of asking "What's Up?". Usually easier to do thru email or text as they can't stop the onslaught!
I think Annie's love-life has all got too much for her, Jane's just told me she was emotionally sick all over her when they met.