She Was Cheating On Him
This was how he found out that his wife was hiding a double life from him. Fair enough, anyone would have gotten lonely in her shoes. However, this was not an excuse for betraying his trust. She was quite good at hiding secrets, but Thomas was not going to take this lying down.
Unable To Forgive Her
As you can imagine, Thomas was reeling with the shock of the betrayal. He could not even think about forgiving Iris for what she did. Instead, he decided to file for divorce. None of this was easy for him, however. We can totally understand why he was very depressed after the discovery.