If she refers to it as "her masterpiece" that sounds to me like it's the way she wants it. If she asked for your advice about the backing, or if you were only discussing the quilting aspect of the piece, that's one thing. She chose the backing she wanted and if you put doubts in her mind and she changed it, then the change worked against her, you'd be the bad guy. Just my opinion, unless she seems unhappy with what she has, leave it be.