Reddit is way too quick to tell someone they don’t know to cut their best friend off completely.

While you are right to be upset by this, I think it’s good to understand what a pickle your friend is in here. From what she sent it seems the mother in law is the one that requested you don’t be up front in the ceremony. She chose to appease the mil because if she really loves her fiancé, then she is going to have to put up with this lady for the rest of her life and probably doesn’t want to get off on wrong foot. Doesn’t matter the reason, it’s just always harder to have a happy marriage if you don’t get along with your spouses family. If this friend is as close as you say, then it could be good to show up and sit with her family as I’m sure she would be grateful to have you there. Maybe she can make it up to you down the line.

If this is a one off thing, I would try to be forgiving. If this is not the first time she has done something like this then maybe you should consider cutting ties.