What is the Halacha regarding attending my friend’s marraige ceremony (with a rent-a-rabbi) to a Schiksa?

The Halacha prohibits giving encouragement to those who sin, and certainly attending the ceremony of someone marrying a non Jew fits that definition. Generally, orthodox Jews avoided such ceremonies, and continue to do so, since the marriage to a non Jew is a denial of the basis of Jewish religious identity. Some Rabbis permit attendance if the purpose is to maintain contact that might ultimately return the wanderer to the Jewish fold. This lenient position is more common 1. for a family member when 2. it is the bride who is Jewish, so the offspring would be Jewish. This is on the theory that family members might be able to influence the children of such a marriage to return to their roots. I believe that generally attendance should be discouraged, but that if you feel an overiding reason to attend exists, you must discuss it with a Rabbi. It may be more appropriate to miss the ceremony but attend part of the reception. OTOH, the reception may not be kosher, and have behavior that itself would preclude attendance.