Hello, VTam. I don't know anything about zither music, for example, what your friend means by 'chord sheets.'

However, googling "zither sheet music" brings up promising sites with music that resembles piano music. Why not have her ramble through those sites to see if anything looks helpful.

Another idea is to search for the company that provided the the pre-bought songs and see if it can sell her more of them.

I'm happy to hear that she is enjoying her instrument.