Is it forbidden to reveal a haram relationship between a man and a married woman to the man's fiancée, who is unaware of it? And what is the expiation for this sin, if it exists?

It is not permissible to accuse an innocent Muslim without clear proof. Believing rumors is not characteristic of Muslims, as Allah Almighty says: "O you who have believed, if there comes to you a disobedient one with information, investigate," and "Avoid much [negative] assumption. Indeed, some assumption is sin." If there is clear proof, or if the person admits to a forbidden relationship, he should be advised to repent. If he insists [on continuing it], then informing his fiancée about it, so she may be wary and deliberate her decision, is part of the commanded advice and not among blameworthy backbiting, provided that it is limited to what is necessary and the other woman is not mentioned.