Just ask her?
A friend of mine is in a similar situation, he's one of those pagans that goes out of his way to show everyone he is, and his fiance is Catholic. [not an extremist, though her family is] she's not 100% ok with his beliefs but she's agreed to observe, and/or read the history, but she doesn't want the kids raised Pagan. Word of advice, don't have her first holiday be Samhain. [My friend took her to our Samhain circle and she was half convinced we were worshiping the devil because of the black, purple and orange decorations, tarot readings and all the spirit communications while in circle.] Ask her to join you for Yule or another 'Christian like' holiday. [Which was her second circle. She was far calmer.]
Also, don't force her to participate, just ask her to watch. Also, don't be upset if she wishes to watch from outside the circle, or she chooses to not participate in the cake and ale. I've had many friends and family observe the circle, and some feel more comfortable outside the circle, or don't wish to participate in any way. It's perfectly fine, don't tell her she has to do anything and it should go over better.