Sorry, but that's rotten advice -- and let me explain why - while we were helping out some kids, we met a father who sadly told us that he wished he had been adopted. We found out more about his (the father's) situation and believe me, the father was dealt a terrible hand - 99.99% he would be at the bottom where he is. He wanted something more for the child and saw that the mother didn't want the child (but he did), so he accepted the child being placed in a loving home and the child now calls him uncle (it probably breaks the man's heart, but he is doing it for the child).
Minding your own business when it comes to an adult is 100% accurate, minding your business when the child is abused is 0% accurate, but this case that I describe is in-between. I do not, do not want to involve the authorities and see no desperate need to (the child is not being abused), but I believe the advice of the ladies in post #12 is better - help the mother with prayer, support etc.