If the Christian girl friend simply fears being tricked into worshipping some alien god, she needs to learn more about her own faith and/or should consider that her partner is offering to share elements of the culture that informs his beliefs and behaviors. If these “rub the wrong way” – better for both to find out now….
The Jewish bible (Tanach) has the same content as the Christian OT, with the books in a different order after you get past the Torah (1st 5 books.) Today’s “Jewish Holidays” are identified in the OT as “Feasts (or “holy appointments”) of the Lord.”
Jesus/Yeshua celebrated these all these feasts throughout his life, plus Hanukah, which came later and celebrated the rededication of the Jewish Temple after pollution by pagans. Hanukah today is mainly an occasion for joyful dining with other Jews/family, and for hope that the temple can be rebuilt on the temple mount. In the NT, Hanukah is referenced as the feast of “Dedication (of the Temple.” Some Christian groups celebrate the holiday as an opportunity to re-dedicate themselves to their faith (their bodies are the temple…)
Jews and Christians worship the same Eternal Being. Jews who’ve read both Testaments argue over whether a) Jesus (a Judean Jew) was “the” messiah (only son of God, and yet God) or b) just another rabbi/teacher, or one of a series of human messiahs targeted to deliver a holy correction to a people gone astray…. The OT identifies the One God as Yehovah, or Yah for short, In the NT, book of Revelation, John identifies Jesus as Jehovah. Decide for yourself.
The Seder (program) was devised by Orthodox Jews, philosophical descendants of the first century Pharisees. Jesus taught that the Pharisees sometimes created traditions that effectively produced the opposite result than what was intended.
Jesus taught the originalist approach to scripture, and the Pharisees who came to believe he was right became some of the first and most ardent Christians. Jesus was closest in attitude to today’s Karaite Jews (scripture only; no required books of rote prayers or rabbinic interpretation.) Today’s atheist Jews are akin to the Saduccees of the first century.
Messianic Christians celebrate the OT holidays/Biblical feasts) but they generally incorporate familiar Orthodox/rabbinic traditions (kippas, prayer shawls, prayer books) into the mix to make believing Jews more comfortable in a Christian setting. You can find good Christian seders on the internet; they present the Exodus story as parallel to the Christian salvation story. You can do your own passover service for family and friends. What a concept.
“The Last Supper was a Seder” – is arguable. It certainly has elements of a passover – but the timing is tricky. Hard to both be and eat the passover lamb. Plenty of arguments over timeline and significance online. This is fun for those who like to argue, but there’s no definitive answer in the Bible. We won’t know until we get there if there’s a there there. Meanwhile, keep the faith!