My cousin wants dioverse with her cheating wife

Dear sir one of my cousin want divorce from her wife. They are married since last 8 years. some six month ago we came to know that her wife is in contact with several boys. we knew the same after numbers found in Vodafone bill. After contacting them we came to know that she was in continuous relationship with them. She had even visited a guy at Poona to meet him. overall we found that she had relationship with more than 10 person. After knowing all the matter she was sent to her parents place but she is not realizing the same. she have been married since eight years. she is mother of two boys respectively 5 years and 2.5 years. Now she is threatening my cousin to file a case for domestic violence and dowry. The boy are ready to give written statement that she was in relationship with them. They are ready to give permission to even record their confession. How we can prove her as cheating wife and how my cousin can get divorce. What legal proof are required to prove her cheating wife ? what are the rules for children custody ?