I guess I’m confused, unless there is some question about whether or not this person is in fact lying, rather than being mistaken, possibly not having said or done what is reasonably suspected, possibly only taking time to disclose an uncomfortable truth at the right time, or the like.
Excepting considerations of that nature, why would you have any obligation in charity to cover up someone else’s lies with your silence? You might decide it is better to tell the person choosing to deceive that you know they are lying and give them a chance to tell the truth on their own. I don’t see how you can collaborate with a lie by tacitly saying nothing either to the person being deceived or the person perpetrating the deception. You do not lie, but you especially do not lie to your spouse. Is someone here afraid to lose the affection of someone who would shun them for not giving them the “privacy” to lie to the person they hope to make their spouse?
I don’t see how you can defend having to say later: “Yes, I knew he was lying to you and I of course understood that you might not want to marry him if you knew he would lie to you about such a matter or if you knew the truth he was hiding from you. I chose not to tell you that you were being lied to, and allowed you to marry him because no one spoke up and told you what you had a right to know.”
I think it is fine to disclose this with an admission that the matter is small and might not make any difference. Still, I’d feel I need to be willing to be disliked by this couple because I disclosed a deception rather than watching someone get married with less information for their choice to give consent than they ought to have. The decision to marry someone is a very grave matter, and that’s how I’d present my decision to disclose the deception. What they decide to do about that is up to them. We may not disclose the faults of others to those with no right to know about them. You can’t tell anyone but the couple and other people affected by this choice to lie. If someone has a fiance that lies to them unbeknownst to them, however, that someone has a right to know.