Kids Custody

Hello i am writing on behalf of my friend who is very stressed and don't know what to do.He is married for 9 yrs with twins 6 yrs.He is a doctor by profession,his wife is a doctor too.She was a divorcee when he came to know her and became friends and she was always sad saying who will marry her since she was feeling all alone. My friend thought of doing good to somebody and decided to marry her since marriage happen after some yrs everything change things started deteriorating whereby she showed more interest in work and always use to doubt him a lot,she is always rude to everybody by which made my friend lost almost all his friends and never have a visiting at home with everybody she use to belittle him and taunt him.He was so stressed that he started working day and doing night shifts as well just not to be around at home.He was having a mutual divorce. Since last few months he started talking to the aupair girl that came to live with the family to take care of kids as he and his wife were busy due to their jobs.Slowly kids and the girl shared a nice bonding and he use to admire that girl and felt better by talking to her sharing things just like a good confident and finally share he shared his feelings for her with her.At first the girl said no because he is married and have kids which will result in problems later but my friend was going in a state which was leading to depression and would face problems even at work slowly even that girl started to develop feelings as she already love the kids and kids love her too. They even shared physical relation as well after couple of months confessing their love.when his wife got a letter which was written by the girl she took a pic of that the letter just state that she is unhappy by the presence of the wife when she come to this house and use the rooms and all.Ultimately she took the twins away from their school without informing to her friends place.When he went there to get his sons back he explain that it was something which was one sided and the girl just wrote her frustrations on the paper after hearing a no from him and he told the wife he want all of them back home when she came with him to the home the girl felt insecure and deceived and she disclose everything to the wife regarding our relationship and all their shared messages where his wife's friend took pictures of it and she withdrawn the mutual divorce consent now blackmailing him to charge in adultery and fight for child custody .Now in that situation he said he will accept all that she wants she is not giving any divorce everyday taunt him and checking his phones emails everything and again and again blackmailing him that she will keep kids away from him and she will see him in court if he does not agree to what she says.Now she want all bank statements all his property papers everything to show to her lawyer.She kept the kids and allow him to see them but i stay away from them in the house and she lives with them in an apartment. His only question is shall he be able to get the custody of his children if she files the adultery case? he loves his kids a lot and always have been like a mother to them most of the time since birth as after some months the wife shifted to another place to work. He is in distress and always asking for advice as he feels helpless and suffocation in the relationship but also want to be with his kids.