In Chapter 48 Mr Gardiner, in London with Mr Bennet, writes to his wife to report the lack of progress in the search for Lydia. (More than Mr Bennet bothers to do for his wife or daughters). In a postscript he says he has written to Colonel Forster to ask him to try to find out from Wickham's friends in the regiment whether he has any relations or connections who might know where in London he is hiding. The he adds "But, on second thoughts, perhaps Lizzy could tell us what relations he has now living, better than any other person."
The ON tells us that "Elizabeth was at no loss to understand from whence this deference for her authority proceeded; but it was not in her power to give any information of so satisfactory a nature as the compliment deserved."
In his annotation to this passage David M Shaphard says:
"The "compliment" concerns Elizabeth's earlier intimacy with Wickham; of course, it is more an insult than a compliment to bring up now the subject of that intimacy, and of her impulsive championing of Wickham's supposed virtues."
Well, of course it would be an insult if that was what Mr Gardiner meant, especially since Elizabeth had made her current views on Wickham abundantly clear to both Gardiners in Chapter 47. I think too well of Mr Gardiner to imagine he would allude to Elizabeth's former relationship with Wickham. I think he is alluding to her present relationship with Darcy, which as we discover from Mrs Gardiner's letter in Chapter 52 the Gardiners think is closer than it actually is. Darcy would certainly know all about Wickham's family, and Mr Gardiner knows that Darcy was with Elizabeth soon after the news of Lydia's elopement arrived and that she may have told him what had happened. (We know that she did.) It would be reasonable to suppose that Darcy might at that point have given her any information he had about Wickham's family and connections - and in fact, if he had not decided to take a direct hand in the matter, it seems to me quite likely that he would at least have suggested that Mr Bennet or Mr Gardiner might make enquiries of Mrs Younge.