"Friend"

Today my best(?) friend was talking about how she wanted to get out of doing a presentation and I said she should just write that she has social anxiety disorder on the top of her essay. She said she could get a doctor's note and we were making jokes about it and at one point I suggested she have my sister write a note signed "BA in psychology, MSU," and then my friend said, "And schizophrenic!!!" And laughed. I almost flipped out but I opted for the look of death, which she didn't even notice. Then she said, "Would she even answer her phone?" ...she was making fun of her because she is so out of touch with the world. I just said, "No, she doesn't have a cell phone anymore," and walked away. I realize I've been a bitch to her lately, but she honestly wasn't trying any passive agressiveness. She's not smart enough for that. She was just being totally careless. I have a very wide-expanding sense of humor, but correct me if I'm wrong, that's just NOT funny. I don't joke about my sister, so what gives HER the right to? I wanted to smack her hard enough to make her realize what she had just done and say, "She's smarter than YOU will ever be even with schizophrenia," but for once, I held it in and left the room.