How to get divorce from wife having extramarital affair

Hi All, My brother got married 8 years back. They have two girl children.Elder one is having 6 years old and younger one 3 years old. 4 years back wife of my brother started talking to one guy (Mason ) who is worked for construction of building. Later they became very close and it went for sexual affair. My brother came to know that she is talking to him via phone.Later my brother warned and took phone from her. But she continued the same with a secret phones which are given by the guy. One night my brother in middle of the sleep found she is not there,after some time she came answered as she went for toilet which is out side of house(very common in small towns) Same thing repeated several times and one fine day my brother caught her with him near bath room. My bro unable to catch him and he ran away. Then he fought with wife and informed the whole things to the relatives. Then she promised it will never happen again and then my bro accepted her and they are happy. After some months Same thing repeated again , that she get caught with mobile phone talking to him secretly. This time he took to her home town and left there itself saying that she will not change. But this time also all the relatives make him compromise and asked him to give a chance and he agreed.Some days they are happy. After one year Same thing repeated again , that she get caught with mobile phone talking to him secretly and sending photos to him through whats app from my brothers phone unknowing to him.One day he replied suddenly when the phone was with my bro in whats app.This time My bro took decision that he couldn't accept her.Police gave counselling to her and my brother.But this time she is not even saying sorry for things happened.She not even bothered 1%.My brother wanted to take divorce from her for mental peace even he can look after children.She not even bothered about children. Here Important thing is She didn't has parents.They died of AIDS. No one will come to marry her if they know about it. All the relatives by collecting money they conducted marriage and she tied knot with my brother. She has to be very thankful for all these,but she is doing all the non sense things. She made disappeared all mobile phones caught without having proof. My brother is working as a salesman in one small cloth show room in their town and earning 8K monthly. Please advise.