The answer is NO, you can’t go.
If the person is someone close to you, you will want to explain your reasons.
Try to do this enough in advance of the wedding date that if the couple had no idea that this was wrong, they have time to meet with a priest and sort things out so that they can do things the right way.
If they already know that they are disobeying the Church, then they won’t be able to say, “But I don’t understand why you can’t support me in this,” and if they do say this, then open your Bible to John 14:15, and ask them, “If I say that I love Jesus, but I don’t obey His commands, am I telling the truth?”
Don’t focus on their disobedience (they already know, and don’t care); rather, focus on that they are asking
you to be disobedient, and tell them, “In good conscience, I cannot disobey Jesus.”
If it’s a casual acquaintance, wish them luck and explain that you have a previous engagement. (You do. You’re going to go to Adoration that day, to pray for their souls. Right?

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