How to get a visa if you are not currently working and do not have a permanent residence?

Hi, I have a friend who is American, but is staying in Germany right now. He wants to visit China soon, but in the application process it says he will have to present proof of the place he is living. However, he does not have a permanent residence there because he just finished his job and is now traveling. So what should he give as a proof of his residence if he is just traveling in Germany?

He also just finished his job, so he cannot present a letter from his work unit because he does not have one. So is this necessary?