If this girl is wife material, which by the length of time you've been dating her you probably think she is, this sets a horrible precedent. If I was best man in a wedding I would expect my wife to show barring an emergency and I know I would do the same for her. Maybe it's just a lack of respect or something or maybe you haven't communicated the importance of this event, but honestly if she knows it's your bff and knows you're the best man, she really shouldn't even be considering going any where else that day.