Statutory Warning: This is a very long post. Read it only if you are patient enough to do so.
Milsi has always been shown to forgive Ayesha no matter what she had done to him. Granted he was angry with her for marrying Neev. He even went to the extent of marrying Prachi (whom he hardly knew and whose family he despised) only to get back at her. But he soon forgot his revenge and treated her as his friend, allowing her to speak her mind to him, which most of the times was to try and poison him against Prachi.
Even after he fell in love with Prachi, he did not shun her. On the other hand, whenever she tried to harm Prachi like leaving her behind at the picnic spot or hitting her with the dandiya stick or accusing Prachi publicly of being Ritwik's mother, although he reproached her with his words or look, he was quick to point out that she will always be his friend. He even bore the humiliation of being slapped by her in public although he refrained her from lashing out at his wife.
While he never allowed her any privileges beyond that of a friend even when Prachi and he were going through marital strain because of the baby raaz and Ayesha tried her level best to take advantage of the situation, he always upheld their friendship. However, when he came to know her hand in the kidnapping of Ritwik, why did he react so strongly? Why did he go to the extent of telling her that he felt ashamed to have known her and had her as a friend? Why did he sever all ties with her?
This is my take.
Ritwik's kidnapping was a watershed in Michi relationship. Although their relationship was strained after Prachi's fateful admission to being Ritwik's mom, he could not bear to separate himself from her permanently. That is why although he had signed the divorce papers Ayesha brought to him, he later tore and threw them at Prachi's face apparently to punish her but really to keep her close to him. When his baba returned from his pilgrimage, there was an added reason to keep her home and pretend nothing happened for the sake of baba's health.
Now Ritwik's kidnapping changed all that. The kidnapping brought out the worst and the best side of him. The worst side being that he very nearly did not give the ransom money and the best being that he did and even went to the extent of ensuring Prachi's safety. In doing so he had seen Prachi's pain first hand but had not given much credence to it because he still believed her to be a liar and a betrayer. He was bashed up by the kidnappers but I don't think that really mattered much to him. The follow up of the kidnapping is what really mattered.
Ayesha had told him that Prachi admitted to baba that the baby was hers. He thought his baba's life was in danger because of Prachi and hence asked her to leave the house, something he had not done even when their misunderstanding was at its peak. When he realised even baba was willing to trust Prachi after knowing the 'truth', he was furious that Prachi can fool everybody. This triggered off the entire divorce case.
Although he had brought up the divorce issue quite a few times earlier, he really went ahead seriously with it after the kidnapping and its fall out. Whatever little sympathy for Prachi he had till then was wiped out after the kidnapping and all he could feel was rage at being used as a puppet by the one woman he loved more than this whole world. And he would have very nearly lost her if it had not been for the tape.
Yes, endangering Ritwik's life and his baba's life must have played up on his mind but what really bothered Milsi was because of Ayesha he very nearly lost his wife forever. He put her through so much pain and fear by nearly denying her the ransom money. He nearly sent her away from his home. He silently watched her as her dignity, her reputation and her family's honour took a severe beating at the courtroom during the divorce trial. The root cause of it all was the fall out of Ritwik's kidnapping. When he realised that this was staged by the one woman he trusted over anybody else in this world, the one woman he thought was his friend and confidant throughout, he was completely shattered and was unwilling to forgive such a gross wrong doing even though Prachi begged him to.
If ever he comes to know Ayesha actually tried to kill his woman, he would surely do much more than just shun her.