ARTICLE 6 of the US/German Tax Treaty
Income from Immovable (Real) Property
1. Income derived by a resident of a Contracting State [in your case, United States] from immovable (real) property (including income from agriculture or forestry) situated in the other Contracting State [in your case, Germany] may be taxed in that other State [Germany].

So your client should be paying tax to Germany on the rental income, then claiming a foreign tax credit on the U.S. return.  

Perhaps with all its furloughed employees, IRS assigned more of them to look at Forms 8833 this year.  It's like driving at Autobahn speeds on an American highway.  Not everyone gets caught, all the time. 

Your reference to a "double tax treaty" leaves us wondering what a "single tax treaty" would look like.