"It was greatly my wish that he should do so," he added, "as soon as his marriage was fixed on. And I think you will agree with me, in considering a removal from that corps as highly advisable, both on his account and my niece's. It is Mr. Wickham's intention to go into the regulars; and among his former friends there are still some who are able and willing to assist him in the army. He has the promise of an ensigncy in General -- 's regiment…” (Chapter 50)

I agree Mr. Gardiner is trying to keep Darcy’s participation secret as he has agreed to do and I forgot Mr. Gardiner could also be considered a former friend. These are good points but still, I do not see why Mr. Gardiner would refer to himself in the third person as one of a group termed former friends. There is no need for subterfuge about his identity is there? Mr. Gardiner is up front in taking the credit for the arrangements just as Darcy wanted him to so he needs no fiction to refer to his activities on Lydia's behalf does he? It would be beside the point since Darcy extracted a promise from him to take all the credit for the arrangements in his stead. It is successful, Mr. Bennet tells his daughters Mr. Gardiner asks too little for what he has done in Chapter 49—paid out at least ten thousand pounds. Mr. Bennet does not question the fact his brother had fortune enough to buy Lydia’s credit or the fact that “some” other friends are “able” and “willing” to help Wickham in getting an army commission. Have I misunderstood you? :D