There were some great comments posted here. At this time it is not a good idea to confront her jws parents. Let her accept the fact they are not coming. It is much harder for her because at her wedding even tho she will look around hoping for her parents they won't be there. The realization of them not appearing will set in after the wedding takes place. The fallout will begin. Her hopes up until this point will be that if they truly loved her they will be there no matter what. It won't be until later when she realizes that her hopes were dashed.
Take your time with her. Let her have her space but hug her. If she wakes up crying after the wedding and the honeymoon, take some time off work if you can and take her to a peaceful place. Just walk holding hands and listening, sharing. It is going to be hard because the loss of this day will be so prevalent for the coming months.
No matter how many people are there she will feel alone at her wedding. No pictures with her parents, no dances with her dad, no dad walking her down the isle. No sharing of this precious moment with those that have loved and supported her during her life. A cutting off.
Let me add one thing, those parents will live to regret this till the day they die. They will rehash all this through the years and grandkids. They will start to realize the control the society had on this memorable day. The guilt will set in for the parents. Such a sad sad thing for a parent to do.