I would have asked her for her degree in quilting! AND when she had made xxx number of quilts she could come back and critique mine. AND that she would learn, if she really cared to learn anything, she would already know that there are dozens of different ways to do any technique. Then I would have given her a hug and told her if she EVER expected to have a lot of quilting friends she had best learn how to be more polite.
Next I would have called the young friend, asked to take her out to lunch, and with humor and a laugh, assured her that her cousin would learn soon enough that she wasn't the "quilt police". Then assured her that if she had a quilt that her cousin criticized she could bring it over and I would assure her that whatever she decided to do, IT WAS GOOD!!!