Been there done that.

She was ripe for an affair anyway, before the death of their son. She acted hypocritically, mad at her husband for coming home late but does the same, opting to go to the home of her old boyfriend whom she's smitten with and sharing a kiss. During that time she was away, the son drowned. She then doesn't waste any time running into the arms of her old boyfriend. He should have saved himself any more grief by cutting his loses and dumping her long before the end.