I'll have to help my nephew decide which city will be best for him to exit Europe this summer, too.

What are her options? Are you talking flying back into Philly or ???
It might make a difference.

Example--Boston nephew is going to Tel Aviv this summer early June. Options are JFK or ATL.
Loads are much better via ATL (and JFK is almost always booked in Business class) so he'll have to go BOS/ATL/TLV.
Return could be AMS, BRU, CDG or FRA. Probably best to try CDG first option because he can fly to any one of JFK, ATL or CVG. He can also take the train to either AMS or BRU if CDG is bad.
Since I live CVG he can break here before going to BOS.

Just some factors to consider plus the possibility of high student/backpacker ratio of travelers.
Is there an option for her to pay a fee to change her return city?
Don't forget PLAN B...