Some possibilities (play for her the pieces and let her decide):
Chopin:
E minor prelude (don't let her butcher it!!!!)
B minor prelude
C minor prelude
A major prelude as a warm up/cool down if you pick something exhausting (but don't sacrifice musicality- it's still Chopin)
D flat major prelude "Raindrop" if she's looking for a challenge
Mazurka 67/1
Mazurka 6/1
Mazurka A minor No. 6 (can't remember the Opus)
Mozart:
Any sonata (take your pick)
Fantasy in D minor
Perhaps a concerto once she's more familiar with his style (understanding mozart also helps with Chopin)
Other things I'm thinking of....
Haydn sonatas
Easier Scarlatti sonatas
Bach Inventions
Beethoven Romanze in F major, if you have a violinist or a second piano handy
Grieg Lyric pieces