In addition to the things already cited apparently Mr. Gardiner was not going to say anything because Mrs. Gardiner tells Lizzy that she felt her husband was pleased to get of letter of inquiry because he could now give credit where it was due instead of being forced to keep the credit for himself. Mrs. Gardiner is actually suprised that Lizzy doesn't already have the facts because she thought Mr. Darcy would have already told her about it. In any case, since she already knows something of the matter, her aunt it seems feels that it is okay to fill her in on the particualars.