My gut says "no" but I couldn't get a straight answer from my priest.
Was it that the priest gave a long explanation covering all the ways that your friend's previous marriage wasn't canonically valid? If your friend was baptized and raised Catholic, his previous secular marriage was a serious lapse, and he'd be repeating that with the new woman. If your friend was raised in some non-Christian sect, then it was just a natural marriage, not a sin.
To keep it simple, Catholics should avoid being present at any ceremony that plays make-believe about marriage.