Sim Only Cards in Germany, for business traveller (London)

Dear All
A client of mine travels constantly to Germany for Business. To avoid roaming charges, we wanted to purchase only local sims with a regular network, not like a global roaming card, which is really common in the UK. Apparently not so in Germany, as you are ask to show proof of address. However my client believes that last year it was just enough to present a passport. Does anyone have any suggestion , I need to come back to him this as soon as possible?
Kindest regards,
Ana Clara