Threaten of false dowry act or any other case to hide illegal sexual relation

My friends married seven year back, and have two children. His wife has sexual relation with another man for last three years. When he came to know about this, he start collecting evidence, and found some photograph with that boy and an audio clip of that boy, in this audio clip he clearly stated about this relationship to other women who is mediator of both. When my friend asked her all these things, she initially refused but later she accepted that she had been in this relationship, my friend has this acceptance audio as well. Upon acceptance his father came and took away his daughter. One child is with mother and one is with father. Now my friend doesn't want to continue the relationship with her, and ready for the divorce, because in first year of marriage she also has telephonic relationship with his boyfriend, but my friend forgive her by saying "this is her's first mistake". Now after 20 days of the incident we still trying to solve problem with mutual understanding, but her family threating to file case against my friend, if he dont take his wife back, but still not file any case. Sirs, please suggest what we can do, is anything that we could do in advance, or we should wait for the filing the case. We are very upset, please help....