Navigation can be done free if she stores Google maps for offline use..she just needs to adust the area, as Google will not allow to store all of Germany in one go..so split it..works fine.

As for sim cards...
the question is, will they be able to get their phones unlocked...and will it really work...
When you buy a SIM in Germany, you need to provide an address, which can be their hotel (some providers accept this and they acticate via phone and not via a letter to the address)
the easiest way as far as I know is getting a card from ORTEL
https://www.ortelmobile.de/en.html
in any of the RELAY shops
http://www.frankfurt-airport.com/con...1/search=relay
at Frankfurt Airport.
They can activate the cards on the spot and know how to deal with foreigners who need SIM cards..
Alternative is e.g. http://www.lycamobile.de/en/ or https://www.lebara.de/?isoCode=en_GB which can be bought in many shops at the airport too, probably even at the RELAY too.

May be they could also look into getting a SIM from T-mobile in the US..Their "Simple Choice Plan" includes free roaming in 140 countries..may be a better offer than staying with AT&T anyway..