My brother wants divorce but wife is not ready

My brother got married 5 years ago(2016) and has a 3 years child.Though it was a love marriage ,things were not good from the very first year of marriage. They always have arguments.His wife always keeps doubting.We, In-laws very few times stayed with them. But they always had fight after 2 or 3 days.All thought that bringing a child would make things better but it never gotten better.In 2019 they had big fight and both physically assaulted each other. Things should have been settled legally that time but Then again elders intervened things got settled for the time being .But things never gotten better.His wife always keeps taunting about his profession and family. His wife is working and earns more than him.suddenly she took leave for 6 months and went to her parents' house.In all these my brother got mentally harrased too much .he says he doesn't love his wife anymore.He loves his child too much and staying together with his wife with all continuous fight will have a bad effect on his child.So he wants to withdraw from this marriage.When he tried to talk about all these she accused him of having affairs and returned back.In the beginning she was the wanting divorce but since my said it first ,she took it on her ego.She said she will destroy him and everything.she is always trying hard to tarnish his image in kins and relatives.my brother is living separately for the last 8 months and wants divorce.He is ready for child custody also.but his wife is very much adamant to not to give divorce and threating to file domestic violence and dowry case. What we can do? We are very worried for my brother sake as his health is detoriating day by day as he is very much disturbed.