2nd marriage- wife family threatening for filing case

There is a friend of mine, whose wife got expired due to complication in her 2nd delivery. He has 2 sons aging 10yrs. and 4yrs.(who is being taken care by his married sister since birth). His dead wife’s family somehow convinced him for getting married with his sister in law who is 10yrs. Younger to him which he did eventually, but they hid about her medical health of high diabetics and urinary tract infection from him about which he came only after marriage. Only duty she fulfilled after marriage was to prepare food for his family, except this she had not done anything else at home during her stay at his home. She did not even bothered to take care of his second child, who is still with his sister. My friend and his wife are neither physically nor emotionally and mentally together in this marriage. My friend is mentally and emotionally involved with another lady of his age. His wife has proof of their whatsapp chat messages. His wife went her home for attending marriage of his cousin along with his elder son, after attending she is not coming back now. Her family is neither talking directly with his family and with intervention of some relative is now telling that they would be filing case against him. They are claiming that they would drag his mother (joint family) and his married sister in the case. I want your guidance as he is very stressed and is not able to understand what exactly he should do.