that's crappy and totally out of line.
I would maybe mention to her that that is the same colour as the BM dresses (which you know she already knows), but perhaps suggest if she could get the same dresses in a different colour ... maybe plant in her head that she won't want to look like she's part of the bridal party, but rather being the groom's mother, she should stand out, not blend in... pump her up that way... ?? Good luck, i would be PO'd too.