The teshuva begins by noting that inviting the non-Jewish spouses could be seen as given a seal of approval to the intermarriage, and such considerations should be kept in mind.
In terms of the issue at hand, there are several ways in which a non-Jew could be allowed to partake of a meal with a Jew on Yom Tov:
1)If the Jew who was cooking did not take the non-Jew into consideration or did not know that he was coming at the time of the cooking.
2) If the cooks in the kitchen are non-Jews and the Jew in the kitchen is simply overseeing the cooking so as to avoid a problem of בישול עכו"ם.
3)If the food is prepared in advance and is merely being warmed up on Yom Tov, and thus no cooking is taking place.
Beyond those issues, the non-Jews at the seder are allowed to be served and can be treated as any other guests once they are there, although they should not be given the matzah that is used for the mitzvah, out of respect for the mitzvah.