It would, indeed, have been unkind to bring up the Thorpes if Mrs. Allen realized it would be hurtful, but I wouldn't put it past her to just not have any idea what had gone on in that quarter, or to not realize the effect it might have had on Catherine's feelings toward the Thorpes (and she probably didn't know that the Tilneys were involved at all). After all, why wouldn't Catherine still like Isabella? She still wore fine muslins, didn't she? ;-)