My wife and I, both fairly observant Jews met a Jewish couple on our honeymoon 25 years ago. We live in different cities, but over the years we have remained friends. We have been invited to and accepted an invitation to their son's wedding. He is marrying a non Jewish woman on a holiday which is also Shabbat. I am deeply conflicted emotionally, spiritually and religiously re: attending. Please advise.
Intermarriage is obviously something very upsetting to traditional Jews, and your friends probably didn't have an easy time with their son's decision.
Conservative rabbis are absolutely prohibited from participating in any way in a mixed marriage under penalty of being expelled from the Rabbinical Assembly.
As a lay person, the decision is yours to make. If you have no choice but to attend due to your close relationship with the groom's parents, you might chose to be at the reception as a mark of respect for the parents and not at the ceremony. You could also go for a short time and then gracefully leave, thus making clear to the family, that as much as you appreciate them you are deeply upset about the intermarriage.
Let us hope you are not faced with this dilemma again in the future.