Can a non-Jewish woman attend a Pesach seder?
Does it make a difference if she is studying to become Jewish and has renounced her born religion?
Does it make a difference if the Seder is in a private home or in a synagogue/hall and open to all who pay in advance?
It is permitted to invite a non-Jewish woman for Seder Night, in particularly if she is studying to become Jewish.
See also sources, below.
There is a general prohibition against inviting non-Jews for Yom Tov meals, which is because of a decree, lest one cooks for him on Yom Tov (which is forbidden; see Shulchan Aruch, Orach Chaim 512:1).
However, in certain cases Chazal were lenient, for instance where he came uninvited. This is certainly true today for the night meal, which is never cooked after nightfall, and everything is ready in advance.
In particular, one can be lenient for a non-Jew studying to become Jewish, as we find many matters for which we are lenient in such circumstances (see Minchas Yitzchak Vol. 3, no. 8). Another special leniency is for a housemaid etc. (Rema, Yoreh De’ah 113:4).
Rav Shlomo Zalman Auerbach also records that the custom of old was to invite non-Jewish dignitaries to the Pesach Seder; this was apparently important to promote good relations between the Jews and their non-Jewish neighbors or hosts (see Shulchan Shlomo, Yom Tov Vol. 1, p. 207, note 8).