Is a religious Jew allowed to go to the wedding of his Jewish friend if his friend is marrying a non-Jew?
Answer:
There is no “formal prohibition” of attending an "intermarriage wedding," 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 any cost. To some degree, presence is a expression of legitimacy. When Ahmadinejad gets up to speak, delegates of morally upright countries leave, because they wish to avoid any 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.
Therefore, although there is no formal prohibition, it is correct to avoid attending if possible.