Question:
Answer:
If the adultery was a generally known public fact, I would not go to the ceremony. I would send them a gift as an indication of a desire for their happiness. At most you could go to the reception, if there is a really compelling need for you to do so. You are free to socialize with them as a couple, especially if you desire to draw them to the Faith and to conversion of heart. Perhaps the gift could be something edifying, like a nice Madonna and child by one of the great masters (there are lots of reproductions, say of Rafael or Murillo). Usually non-Catholics have no objection to something like that.
Simply, you should not seem to approve of their adultery, but you should still show that you love them and are open to them in view of their conversion. Obviously, you should be praying for them, for this intention, and for their families.