ANSWER:

You have already lost her when she decided to marry a Kafir! Her father must prevent this marriage. If she marries him anyway and thinks her marriage to be legit, she has left the fold of Islam and is an Apostate. People must know this to cut her off their society and to be deprived from inheriting or being part of the family. If she believes that it is prohibited but still goes ahead with it, she is a fornicator and an adulteress and it is absolutely prohibited for you to attend this so called wedding. 

My daughter is marrying a non-Muslim without our consent. It had been very tough but now it’s like we’ll loose our daughter when we refuse to attend the wedding and she’s taking it very personally and saying that we are cutting off ties though that’s not our intention. My whole family is practicing Muslim but it’s so confusing that I’m getting mixed advices regarding attending it