Permissibility of Attending Intermarriage

I’m in an awkward unexpected position. I am scheduled to be the maid of honor at the wedding of my best friend since elementary school who is not Jewish. Even though we are different religions, we consider each other to be sisters.
She made a point of scheduling the wedding on Saturday night after Shabbat ends so I can be there. It is going to be a civil ceremony. I am all ready for this wedding, having bought a dress and had my name printed on a program all at the nonrefundable cost of hundreds of dollars paid by her parents. She is looking forward so much to having me in that role.
Just this past week, I learned to my surprise that the man she is marrying, who I have met infrequently because of his busy work schedule, is actually a Jew according to Jewish law, even though he doesn’t consider himself one. I found out by accident that his mother is a Jew who was brought up Jewish and converted to Catholicism when she married his father. His immediate family actually practices very little religion at all and he considers himself an atheist.
In other words, this wedding is an intermarriage. I never would have guessed he was Jewish because he doesn’t have a Jewish name or looks. They way I found out is I attended a party held by his family and was surprised to see other Orthodox Jews there. One of them told me she is his cousin and explained how they are related. Not just that, but she says she will be at the wedding and doesn’t mind attending and they are getting kosher food for her too.
What am I supposed to do here? Can I still go to this wedding? What should I tell my friend?