Dear Soliloquy:
I think your friend needs some mainstream repertory, doesn't s/he? So, the obvious would be:
Baroque: Bach;
Classical: Mozart or Beethoven earlier works;
20th century: Debussy, Bartok, Schoenberg, Prokofiev, Ravel...OK, there are hundreds of possibilities here. Just take care of one thing: some works by these masters were actually written in the 19th century (the Suite Bergamasque, for instance, is from 1890).
Although Scarlatti usually fits, I dislike the fact that is largely considered a Baroque composer, because he's not: a great part of his output is Pre-Classical. So, a bit of discretion when choosing a work by him is very welcome.
Best!