Focus on yourself first. Be patient, patient, patient. Show understanding and respect. Give her lots of positive encouragement no matter how bad she does. Laugh on the inside when she falls, not on the outside.
I've posted this here before: Foster this and you'll have a snowboard buddy for life. I don't know how experienced she is, but a private lesson is also money well spent. Have patience and don't yell at her. I've seen guys literally call their girls bitches, useless fucktards, and many other derogatory things on the slopes while trying to teach them to board, because they're losing patience. That's not how you teach someone to board. Patience, politeness, respect, compassion, and understanding should be your mantra here. It's worth every second of your time once she starts joining you on the slopes.