He mom should of went to the dressing room with her, or at least waited in the area. Kate would of never walked out of the dressing room area. The scene was written stupid.
I think most moms would have gone with their daughter and wait outside while she tried the dresses on. But Kate and Rebecca don't really have that kind of relationship. Kate is moody and lashes out. Rebecca gives her space. I don't believe Kate would have wanted her mom around her and I think Rebecca knew that and purposely gave her distance. Kate would have likely scoffed and made her mother leave the dressing area. Rebecca was shocked Kate even asked her to go with her to the mall and was trying to not mess it up. At least that is what I think.