Is informing the suitor that his fiancée loves someone else considered trustworthiness or forbidden backbiting?

The principle is to conceal a Muslim's flaws and not spread them or accuse them without evidence, as Allah Almighty says: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other." However, if there is an overriding benefit in revealing some flaws, such as during a marriage proposal, there is no harm, provided that it is only to the extent of the need and with certainty of the flaws' existence.