I'm REALLY confused.
Question: Can I attend his wedding?
Here's an answer from Catholic Exchange:
But what if a Catholic were getting married outside the Church? This is yet another very serious question that many Catholics wonder about today. Can a Catholic attend the wedding of another Catholic who is marrying outside the Church? Although there is no legislation to prevent a Catholic from attending such a wedding, in the document Orientalium Ecclesiarum (no. 26) of the Second Vatican Council, we learn that divine law forbids any common worship (communicatio in sacris) which would damage the unity of the Church, or involve formal acceptance of falsehood or the danger of deviation from the faith, of scandal, or of indifferentism (a tendency to think it makes no difference what we do). Therefore, whether or not it is wrong for you or any Catholic to attend the wedding of a Catholic marrying outside the Church depends on the possibility of your presence or participation causing such things to occur.
Many Catholics and other Christians, for example, do not understand the seriousness of the sin of scandal. But Jesus Himself warned, “Whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened round his neck and to be drowned in the depth of the sea. Woe to the world because of things that cause sin! Such things must come, but woe to the one through whom they come!” (Matthew 18: 6-7). “Scandal is a grave offense if, by deed or omission, another is deliberately led into a grave offense” (CCC 2284).