Extramartial affair

Legally he can not seek separation ( divorce) . Your friend can also create lot of problems for the husband but she in the process shall also suffer. she should make efforts to win back her husband before taking any steps advised herein or otherwise. 

in matrimonial disputes, the spouse, who is emotionally and economically weak will suffer more.

Foolish man. He's lost everything. Let him have his so called affair ,in fact hurt his ego and tel him that you want to get rid of him and would like a divorce as you want to marry your lover[FAKE THE STORY]. You won't lose anything[DON'T GIVE HIM ANY THING IN WRITING or recordale in spy device by him]. In case he does agree to get a divorce, go to the court and tell the judge that "He's forcing me to divorce him since he wants to marry his mistress". All male judges take the side of the woman even if sh'es wrong. However Thukral is right, he cannot seek seperation. Ask yourself one simple thing: "If he's so madly in love with the mistress/whosoever, why he sought seperation and not divorce? Simple: HE DOES NOT WANT TO LEAVE YOU.

What's his role in children's life?

His role in children's lives will begin in right earnest when the married woman he "loves" dumps your husband since she has to appease her kids if any as well husband who should be more demanding. Meet the married woman tell her this "This guy(my husband) is a pain in my neck and forcibly married me since no women wanted to marry trash like him. I too would not have married him had I seen his mug(face). I saw his horror face only on suhaagraat and screamed. Thank God you have him well  hooked. Now we are both women and must help each other. I want to help you by making him marry you. So you divorce your husband and marry this joker(Aggrieved's husband) so that this piece of trash gets out of my life".

IN SHORT: USE THE QUALITIES BESTWOED ON ALL WOMEN BY GOD WHICH ARE SUMMARISED BELOW:

"Treachery, deceipt, lying and gold digging thy name is woman"

He loves his children dearly. He has 2 girls. But the children are attached to the mother.

@Promodini: There are no legal hassles. TAKE MY WORD FOR IT, the married woman will dump him as HE IS BOUND TO GET BORED. GIRL KIDS WILL BE ON HIS MIND but when he he comes,treat him like a doormat even though he is "IMPURIFIED HMSELF". A better add on. When he becomes your doormat, you keep a boyfriend and he'll still not mind to be doormat. Try understanding male psycology.