Is it halachically permissible for a Jew to be present at a wedding between a Jew and a non-Jew? If not, what is the source for the prohibition?
It is halachically prohibited to enter a non-kosher restaurant, even without the intention of eating. The reason for this is maris ayin — giving the impression of a transgression is also prohibited. In the case of intermarriage, I don’t think any “formal prohibition” exists, but the question is the impression that will be given.
An intermarriage is one of the gravest things that a Jew can conceive of; it implies a tragic loss to the Jewish nation, and is something we must seek to avoid at all costs. To some degree, presence is a personal expression of legitimacy. When Ahmadinejad gets up to speak, delegates of morally upright countries leave, because they wish to avoid an expression of legitimacy. As a general rule, the same would apply to the Orthodox invitee to a mixed marriage: at some level, his presence is an expression of legitimacy, and it should therefore be avoided.
In addition, we find in the Gemara a general instruction to steer clear of “bad things” (e.g. Avodah Zarah 17, concerning idolatry or a house of ill repute). The same might be said of the wedding in question.
Sources: Iggros Moshe, Orach Chaim, II:40 (concerning maris ayin).