My niece has down syndrome and a problem :/

She is 3 years old and she doesn't talk well at all and she signs a little. She is my world my everything but.. she doesn't go on the potty yet. Lately she has been sticking her hands down her pull up and pulling the poop out and showing people and even eating it!!!!! oh god it's terrible! Her mother is so stupid and wont teach her how to go on the potty and I just wonder why she would do something so gross. You think she'd stop after the first time she tasted it!!!! EWWW help!