How can I get custody of my friends son
Do you honestly believe you'll make a better mother? Why not have your own? Also the only option is if she's abusing her kid is to call Child Protective Service and they may or may not take her kid away for awhile and leave it in the guardianship of her parents or grandparents until she can prove that she is a fit mother. And if after months she cannot prove it her immediate family will get first dib's on adopting the child and let's just say that her family didn't want it either then you would have to go sign up with an an adoption agency and prove that you would be a suitable parent and more than likely they would adopt the child out to a married couple before a single 21 year old girl. So I'd say the chances of you getting custody are slim to none so don't bother trying.
Can you explain a little bit more about the situation please. You have not given us enough to advise you with.
There is not enough information given to be able to give you an an answer and it really all depends.