It sounded to me as if he was talking about an actual piano (where the only wires are the ones that go "boing" when they're hit with a hammer!). I saw one of these on a streetside garbage pile the other day, but didn't have a car I could pick it up with.
In which case my strategy would be to devise some sort of momentary switch that's triggered by striking the keys - if the hammers are intact, you could use those as well (and it'd look better and be classier). Apparently velocity-sensitive keyboards use two switches, and time the difference to determine strike speed/intensity - but that's another level of complexity. The switches themselves are the critical component, because they'll affect how the piano feels to play.
Get yourself a synth keyboard that works (yard sale, swap meet - no more than $20 for a starter; usually a few of the keys or the key buttons are broken and you won't need those), tear it down and trace where the keys go. If you can duplicate/wire into this you don't even need the Arduino since the keyboard chip does all the matrix scanning as well as synthesis.
Wire the piano keys to match the synth matrix. The (low-end Casio) synth's I've looked at use an 8xN matrix so key 1 would connect pin 1 to pin 9, key 2 would be 2 to 9; key 9 would connect 1 to 10. 8x5 is 40 keys and in the keyboards I'm using this is typical. (The "voice select", volume, etc keys use similar matrix connections).
Failing that, build yourself a trebuchet. You'll need several telephone poles, and a Volkswagen for a counterweight...