Because they are unhappy with themselves... So in return they try to bring everyone else down around them. Cut yourself from them and watch your surroundings grow. They are a bad apple
Well because in general people are bad in these days and they made this things .You should be very careful when you choose the person you stay with as they dictate your future
Just share your views about her way of talking with her gently and tell that you don't like the way she talks about yourself in front of other people, if this solves your problem then that it is Ok for you or else say her that i don't want to continue my friendship with you due to your this behavior. Then see how things comes to you .....