I would comment on the colors or the contrasts, things like that.
Egos are tender plants and must be nurtured.
BUT should anyone say their quilt is going to a judged show and wanted any criticism, then it should be an honest answer. That one small defect that her mind hasn't seen, might get her quilt passed over. That's what she/he wanted, advice and that is what she/he honestly asked for and should get.