There is a saying in Kannada, MULLANA MULLINDANE TEGIBEKU... mean to say, to remove thorn one needs to use a thorn. Getting divorce under Indian law is not easy. You file divorce, on grounds of adultery, if you fail to prove it, wife can file case on you and get you behind bars, that is the pathetic situation in INDIAN LAW..
1. Go for Mutual consent divorce.
2. Or ask your son to also have extra marital affair.
3. or seek intervention of her parents and sit and talk and decide future course which is explained above.
4. If s*xual concerns are there, better ask son to seek medical help. if sons wife is having pschos*xual problems, take her to a endocrinologist.
5. or If your son has got pucca evidence that she has slept with somene else, buy a gun and shoot her in the head and there ends the matter, no divorce, no headache, no tension at least other females will learn to be faithful to their husbands.
6. or better find someone else and ask son to live with her. faithfullness is all up there in the head not down there.
Advices provided to be taken at own risk. All the best.