Should the fiancée of a deceitful suitor be informed of her fiancé’s deception, or would that cause discord? And am I entitled to supplicate against those who deceived and lied to me?

If a man reports something contrary to reality, he is a liar, and lying is forbidden and a grave sin. If he intends to allude to slandering his wife, this is a serious matter. His engaging in emotional relationships with foreign women is a reprehensible act and a pagan behavior. He should be advised gently and warned about God. If he does not desist, his case can be referred to the responsible authorities. There is no need to inform his wife to avoid further harm. As for supplicating against him, if his actions have not caused harm, then there is no justification for it, and forgiveness is better.