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I am unfamiliar with the nuances of what occurs in a non-Orthodox conversion. However, I have heard from a few Orthodox rabbis and friends that a non-Orthodox conversion is "invalid" and the convert is not actually Jewish.
I see that this question addresses whether a Jew can attend an intermarriage ceremony. Is there any leniency regarding attending one where a Jew marries a convert from a non-Orthodox wedding? Or does this have the same rules as if this were an intermarriage?