Wife having extramarital affair

File for divorce against your wife on grounds of adultery , mental cruelty. Make lover co-respondent in said case. You can also file criminal complaint of adultery under section 497 of IPC against her lover. However please note that your wife cannot be punished for said offence . usually, there are no direct evidences against adultery and it has to be proven with the help of circumstantial proofs such as, photographs of the spouse with her lover at secluded place where they may get intimate or at places like hotels where they may get an opportunity for physical relationship. 
Since, it is difficult to have an eyewitness to prove an illicit relationship, it can be proved indirectly by showing evidences such as hotel bills or travel records.