As someone who went through exactly this process, I have to correct you with this specific statement. You do not have to file something with the German government before filing for US citizenship. However, you have to be issued the "Beibehaltungsgenehmigung" with the German government before taking the Oath. A small, but vital difference.
He said, that he already had his Oath ceremony. So at this point it is too late for this. He lost his German citizenship. However, upto 12 years after the Oath you can still file for this. The only issue is, that you have to prove ties to both countries at the time before the Oath.
I send the N-400 as well as the Beibehaltungsgenehmigung at the same time. The Beibehaltungsgenehmigung was approved about 2,5 month later. The Oath came about four month after that.
Basti909851 got a reaction from EM_Vandaveer in Green card renewal and filed N-400