Extra marital affair

Hi, my brother is in married life since 18 years and since first day of marraige, my brother and my sis in law had lot of diffrences as my sis in law had attempt of suicide at her wedding day as her marraige was against her wish. My brother tried to adjust & cooperate but his wife couldnt make place in his heart. Now its been 18yrs of their marraige and my brother has an affaire with a divorced woman from past 4 years as he found himself happy with his galfriend. He has 2 children. He has been seprated from his wife from last 4 months (dec 2018). Now he want divorce and want to spend life with new partner. He is fedup from his wife and her family menral tourture happening since 18 years infact his brotherinlaw filled a comlaint in women cell against my brother, family n his galfriend also... my brother's brother-in-law tourturing his galfriend. He distributed his galfriend phone number to his friends who call her and tourture.. infact my brother's galfriend received a call from women cell who were suggesting to leave him and donot come in his married life issues. But we all knows that my brothers inlaws are very cruel n dangerous and we dont like because of their cruel behaviour. My brother is getting sick now. Suggest what he should do...