What is the ruling of Islamic law regarding a woman who claims to be the wife of her ex-husband, who passed away years ago, while she is married to another man?

If a woman mentions that she is the wife of the deceased out of consideration for what was in the past, there is no harm. However, attributing this to the present is a lie, as the marital bond is severed by death. She must treat her husband with kindness, and this includes not mentioning her previous husband in a way that hurts his feelings.