Isn't Divorce a Sin?

Aunty Amara good morning...
I have been following up on all the life stories that you have been posting,so I decided to ask you this question.. Here is the story...
I have an elderly friend,we just became friends towards the end of last month..He is a married man or should I say.."was a married man"...We became close then he told me his story..
He once cheated on his wife years back,he says he asked her endlessly for forgiveness,which she swore to have forgiven,the result of that adultery was a boy...
He said since then,all he does is just to cater for the child's welfare by sending money to him and his mother etc.He hasn't cheated again since then..He has two children from his marriage to his wife after so many years of childlessness...
To cut the whole story short...His wife made him believe that he was the cause of their childlessness, that he had a low sperm count..until of course when he had that adultery child then he began to wonder....
Years later,he discovered that his two kids from his wife are actually from another man...His wife has been working with their doctor,collecting money from him,made him believe the adulterous woman lied to him about the real father of the boy..saying that he has a very low sperm count...
Later,matters came up and the whole truth was reviewed... When he went for a business trip,neighbors called him that his wife was moving out...
He quickly called his people to help him talk to her,all to no avail.. After she left,he got back, tried to beg her to come back but she refused...Even his people went to beg her people and her but all to no avail too....She had left with the two kids...
His parents asked him to forget her that God had revealed to them years back that the woman wasn't the right womañ for him...
He also said he went to God in prayer and God revealed the same thing to him...I have been trying to talk him into going back to his wife but it seems his mind is made up on forgetting her and moving on with his life..He says he even plans to remarry someday...
My question now is....isn't divorce/separation a sin???... Shouldn't marriage be "for better or for worse"?
Am I wrong in telling him to go back to his wife?
Sorry for the lengthy write up,really hope to get a reply soon...Thank you.

Let's extend your questions to help us understand your mail. If the man who vowed to love and protect his wife in sickness and in health, in good times and in not so great moments, ends up killing the wife out of physical violence, please how will she get her life back? 
If there is gross infidelity by one partner which will expose the other to a sexually transmitted diseases and infections like HIV and AIDS, herpes, and the likes which are life threatening to the person, are you suggesting that such a partner should swallow it all even when it may destroy his or her life? 
If a man has children outside and a wife bears children of another man in the man's house, and the owner of the children is always disturbing the peace and progress of the home, please are you suggesting that they should pretend that everything is normal? 
Granted that God hates divorce, the Bible never said that separation and divorce is a sin however it gave terms and conditions of considering divorce. 
Personally when I see that a union is marred with physical, emotional, mental and psychological abuse, the first thing any wise and Godly individual will advocate is separation so that couples can seek for ways to resolve their differences in their marriage and in a situation where such differences cannot be resolved, they may decide to let go of the marriage if they so wish. 
But the God who hates divorce also abhors infidelity and detests killing. 
Many wives and husbands died helplessly and pitifully because they wanted the society to see them as married while in their closet, they were yearning for some time off from their marriage. 
You were not wrong to have suggested he returns to the wife but if he cannot forgive his wife of her flaws, how do you think he can possibly live with her and love her as his wife? 
If the wife also could withhold such a sensitive information from her husband and has made him to falsely believe that he was the father of her children while the wife was being serviced by another man, how do you think that he will cope with those children and her? 
He can only return to her if only he can forgive her and accept her children as his just as she did for him. 
Marriage becomes for better results and worse experience when the partners surrender their home to God and allow Him to guide their footsteps. 
Marriage becomes beautiful when partners humble themselves and let go of pride and selfishness while serving one another with love, understanding and selfless devotion. 
Marriage becomes for better or worse when each partner chooses to forgive instead of retaliating or punishing another, when each partner protects, preserves and provides the need of the partner without bearing grudge or resentments. 
Marriage prospers when partners are totally naked to each other spiritually, emotionally, physically, financially and psychologically without any form of competition or disdain treatment. 
Marriage thrives when partners always remember that their partner is a priceless and precious gift from God who they will be accountable to on the very last day and not objects for shouting, beating, and starving of sexual intimacy. 
While nobody wishes for a doom after wedding, it is always wise to invest more time in preparing for your marriage so that you don't end up with a cannibal or a camelion as a partner.