My friend is a horrible mother. What should I do?

I'm 17 and my friend (Shelby) is 17 as well.I've known her for a good 3 years now and she's one ofmy bestest friends. She's never been to school and just stay home all day sitting around or on the computer. She got pregnant when she was 13 and had a baby (Emily). I've begin to notice lately that she is a HORRIBLE mother. Emily ( just turned 3) just doesn't know how to behave and she always wants attention but her mother doesn't give her any. The only time I ever see her get attention from her mother is when Shelby yells at her for doing something bad. Which is all the time. I'm alway over at their house so I know what's going on. Her grandmother does all the work taking care of Emily. Shelby always hits Emily when she does something wrong. Hard..and sometimes in the face. Is that child abuse? Every time I'm over there I see Shelby hit her. Is she hitting her too much? It pisses me off. She doesn't have a job and all she does is sit around on the computer at home. She has never been to school and she's overweight. Everytime I'm at her house I want to yell at her for being a neglectful mother. Should I tell her what I think? What should I do? Alot of her other friends (and sisters) feel the same way as I do that she's a bad mother.