What rights does a mother have getting full custody?

My friend is going to court for custody decision about her 3 yr old daughter. She now has split custody with her daughter's father, he takes her half the week, & she has her the other half. His live-in girlfriend is absolutely crazy, she swears a lot, screams a lot, she breaks things when she's angry, she starts fights with everyone, she smokes pot every day, she & this little girl's father got pulled over with a DWI & possession of marijuana a while ago. She bad mouths my friend and her roommate on Facebook, sends nasty texts, etc... My friend doesn't do anything back to her. She doesn't drink, smoke, party, she never swears in front of her kids (she also has a 7 yr old son), she takes her kids to church every Sunday, but her ex is an atheist. How do you decide what belief system to go by when both parents have opposite views? My friend is just not ever retaliating. But she wants what's best for her daughter, and doesn't feel that his girlfriend is fit to be an influence around her daughter. Can she get full custody with this info I provided? (NY)