Berlin bike hire

Hi I have a friend joining me from Australia for the last three weeks of my 5 month cycle trip in Europe. She needs to hire a bike for about 14 days preferably a one way hire Berlin to Frankfurt or return to Berlin. So a) any leads on best places to do this with good touring bikes? So far Fat Tyres looks like the best deal; b) any Warm Shower person prepared to lend a touring bike to very reliable, careful woman? Frame size required would be 26 inch for preference may get away with a 24 inch with good seat adjustment. Val will bring own seat and panniers. We are potentially looking at cycling the Oder River route. Thanks.