Is how I read this. Lizzie is aware by this time of Wickham's past behaviour but her Aunt doesn't. From the last page of Chapter 43..

"Elizabeth here felt herself called on to say something in vindication of his behaviour to Wickham; and therefore gave them to understand, in as guarded a manner as she could, that by what she had heard from his relations in Kent, his actions were capable of a very different construction; and that his character was by no means so faulty, nor Wickham's so amiable, as they had been considered in Hertfordshire. In confirmation of this she related the particulars of all the pecuniary transactions in which they had been connected, without actually naming her authority, but stating it to be such as might be relied on."