RamJam:
Take an oscilloscope. Stick microphone in scope. Hit C on piano, measure frequency. Then hit next note and so on up to octave above. Program Arduino to emit said frequency in accord to button pressed.
Right, because it sounds like he probably has an oscilloscope just laying around.....?
Or....google "music note frequency" and find something like this -
http://www.audiology.org/news/interviews/documents/chasinconversionchart.pdf
Then, follow the tone and button tutorials to figure out the rest.