I agree with the above poster, have them take the train from FRA to Berlin if they can get cheaper or better flights in to FRA. The train is easy to use, nice and clean and you get a great view of the countryside. Plus if you get a first class ticket you can sit next to the food car and order beer. I just returned from a week and half in Germany and all we used was the train. Flew into FRA and immediately got on the train and left(spent zero time in Frankfurt other than the airport) and took the train back to FRA on the morning we left.

Oh, and we cleared 3 mileage upgrades(myself, wife, and daughter) for the return trip. Actually they cleared about 3 or 4 days before we departed DFW for FRA.