What should i do for my cousin?

So, a couple months ago, my cousin left her abusive boyfriend, in which she did heroine with, and brought her 2 year old son and herself to live at my grandmother's house. of course, my grandmother is too naive and ignored everything she did in the past. when she was living at the house, she never paid rent, babysitting, groceries, nothing. and took complete advantage of my grandmother. later, she got a job, an apartment, a very nice and respectful boyfriend, and filed for custody of her son. everyone thought she was doing great until about 2 weeks in. when she was done with her job, she wouldnt pick up her son and just go home. then she wouldnt answer phone calls. then she wouldnt contact anybody for weeks. she came to visit her son a couple days ago, she gave my grandmothers phone back (which she stole, and by giving it back no one could contact her), brought all of her sons clothes and toys, and said she didnt have a job, apartment, and had a new boyfriend. days later, her mother called and informed us that she and her new boyfriend were moving to rhode island (we live in new york) is is legal to just leave your son without telling the person? and i dont want her ex boyfriend (her father) to get custody (drug addict). i also suspect she's on drugs too. my grandmother really isnt doing anything because shes attached her grandson, and honestly, she wants to put him in her custody, but she is elderly and cant keep up with a 2 year old. what should i do? im only 13 btw but im more mature than most of the people in this situation. sorry its long btw :)