As we agree, Mr. G has agreed to keep Darcy's participation a secret and take any credit that can be assigned. We also know that Mr. G is uncomfortable about this. If Mr. G knows Mr. Bennet as we know him, he knows that Mr. Bennet is satisfied to have the Lydia situation fixed and--except for clearing his financial obligation to Mr. G--wishes to take as little further interest in the matter as possible. Certainly Mr. G tries to discourage Mr. B from further inquiry.

Mr. Gardiner soon wrote again to his brother. To Mr. Bennet's acknowledgments he briefly replied, with assurances of his eagerness to promote the welfare of any of his family; and concluded with entreaties that the subject might never be mentioned to him again. The principal purport of his letter was to inform them that Mr. Wickham had resolved on quitting the Militia. (Ch. 50)

So in the same letter that mentions the commission in the north, Mr. G is already trying to diffuse (and complicate?) Mr. Bennet's search for an accounting. Since Mr. B may believe Mr. G to have the money for a commission but have no army contacts, Mr. G may not be able credibly to say he got Wickham a commission. If he says "a friend" made the arrangement, Mr. Bennet might ask who in order to thank him (that might take little enough effort and involve no money), but if Mr. G makes a general (if misleading) reference to "some friends," the vagueness discourages further inquiry. I think that was Mr. G's intent.