Older guy here. Is your friend perhaps gay? I only ask because this sounds like a jealousy thing which only makes sense if she is attracted to you and is angry that she is "losing" you. Small things to show she is upset like wearing boots but nothing super bad because she really cares for you.
If you're looking for a way to help her I would treat this like she is that guy "friend" who you know has a crush on you but has no chance with you. Be sweet with her and let her know she has always been a special friend and ask her if after you are married you can still talk to and confide in her as a friend. During the conversation you can mention how much it means to you to have her in the wedding party but this is your only chance to have the wedding of your dreams and how important it is to have her help and if she can you would really appreciate her helping more by wearing the sandals. The make up may be a cross to big for her to bear as I can see it being taken as a subjugation thing for her. i.e.: Wearing a mask to hide herself as though you can't accept me for who I am kind of thing. Anyway even if she is not gay see if these help in any way possible. Good luck and God Bless your marriage.