As a former DC resident (lived there till early '07), my suggestion would be US/UA. The OP did not mention where in the DC area she lives. That could affect the choice as well. For example, for someone living say, near College Park, IAD can be a pain to reach (thanks to the beltway, and the lack of metro connectivity to Dulles). Flying out of DCA is fantastic IMO. If she doesn't mind connecting in CLT/PHX, US would be a good option.

Another point in favor of US is that she can fly Virgin Atlantic and accrue US miles. Of course, you have *A. LH flies to MUC and FRA from IAD. Connecting in MUC, from most accounts, is way better than FRA.