Brother's wife cheating

Good day all, my elder brother is having some problem in his marriage,after enquiring i came to know something is troubling him a lot and finally he told me that his wife is having affair with another man,which was her freind even before her marriage with my brother, they use to talk on phone after marriage but she composed my brother saying, give me some time i will need some time to move on, finally she move to a seprate home out my parent city then now situation is this guy is living with them and when my brother opposes she threaten him that she will slit her wrist and put it on my brother,my brother only concern is her 2 daughters, as per him there is no hope that she will quit this affair, now we are running our horses to what to do and what i need some advices..on leagl ground.. can we proceed for a divorce on mutual concent 2.if we want to prove her character is wrong, then we only have some audio clips talking her n my brother,accepting by her that she will not quit him and she dont care for her childrens,kill them burn them she just dont care 3. they already hav more than 8 years of marriage and this guy is living with them for more than 6 months, so what should we do so that it may not come round on my brother. 4what about children custody, we surely want it,she at no cost give anything good to them 5 she is working in a private firm how can we prove that she is self dependent 6....finally what should we do..