Child maintenance, NOC form, custody

Hello, I'm writing on behalf of my sister in Kerala. She has an 11-year-old son and has raised him alone as the father abandoned the family about 9 years ago. Father decided to move to the US alone and had a decent job earning approximately $50- 60,000 USD annually. After the child was 2 years old he stopped contributing financially, even after devasting floods ruined the mother & chlid's home. He has only lived with the mother and son for less than one year after the birth of the child. They are finally getting divorced after a long wait. My questions: What portion of the father's salary is normal maintenance for the child? The mother earns an Indian salary equivalent to $15,000 USD annually. Second, how difficult would it be to obtain an NOC form? It does not seem fair that the father can abandon the mother and child, move to the US, make a decent salary, and then restrict the travel and possible relocation of the mother and child. This seems like she is being penalized for raising the child and not being allowed to leave the country for her career and a better life for her and their son.