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"Beiug a maid of honor at a wedding does not mean that you officially endorse the marriage."
Actually, depending on the church the friend is getting married in, it can. I got married in the Episcopal church, and the idea behind being a maid of honor/best man, being a "witness," was that you were officially endorsing the wedding. I do not know if that is the case for other religions.
I think Cary's advice is spot-on in this case.