What is the ruling on someone who tells a married woman that an unmarried man is speaking ill of her, and deceives her into committing adultery with him under the pretext that he loves her?

If the informer is a liar, his act is a great sin and a reprehensible crime. It is a slander for which Allah has threatened its doer with entry into the mire of Khabaal.

However, if he is truthful, he will be rewarded, because advising Muslims is obligatory. This situation is an exception to the forbidden backbiting, for the purpose of warning Muslims from evil, and it is permissible by consensus, rather obligatory.

As for a person who speaks with a woman with the intention of fornication, he is a wicked transgressor, and his act is forbidden; because means take the ruling of ends in Islamic Sharia.