Child custody after mutual consent divorce

My friend is in a bad marriage with her in-laws & husband. She & her husband have a girl child of age 7. The husband does not provide any money for the child's upbringing & stays away from home and also abuses the wife. So after all this, the wife left her in-law's home and child to go work far elsewhere and has been living separate away for 3 years working and sending money to her evil in-laws who keep the daughter. The abusive husband doesn't send any money for the child. Only the wife does. The wife doesn't want to be in the marriage anymore and is looking to divorce him & gain sole custody of child. But the in-laws and husband are stubborn regarding the child. The in-laws are very old & live on pension & the husband doesn't spend any money on the child's upbringing. How can she proceed with this divorce? She can get mutual consent divorce because her husband & in laws don't care for the wife. But they will aim to keep the child and not give it to the mother even after mutual consent divorce. Kindly help me with this issue. Both are hindus & marriage was only solemnized at home & there is no marriage certificate because they never registered it. She is in a very bad marriage. Kindly help me with any good ideas pls. Also the fastest possible divorce route will be best because of the violent husband.