In the DB world, it is recommended that you not "out the affair" and allow it to die a slow death on its own. Why? Because if the affair is outed, it will make it more difficult for the wayward spouse to want to come home if he/she knows that people, especially family members are aware of the circumstances.
Also, outing the affair could very well push the affair partners closer together so that they can support and protect each other from the negative thoughts and actions of others. They will do everything they can to prove that everyone is wrong and that they are right about being together.
Generally, when the affair dies a slow death, they have no one to blame but themselves and then there is no resent/anger towards someone who has outed them, therefore there is no longer a need to justify why the left.
Of course, it's up to you as to whether you want to sit quietly and allow it to die a slow death, which could pave the way for them to return home without having to face everyone along the way; or you could out them and push them closer together.
In my opinion, I would sit quietly for a bit because the affair will most likely come to light all on it's own. Again, that's your decision as to how you want to handle it. I've seen both ways take place and generally sitting quietly has a better chance of working out.
Just my two cents.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
