You can use solenoids, no problem. The main problem will be that you need a lot of them.
I don't think it's a good idea to put them on top, pressing the keys. That would be ugly/cumbersome. I'd hide them away underneath and make them pull strings or something like that.
You also need to be able to play expressively. You need to think about PWM driving the solenoids to pull the strings with different force.
(...and does he really want a big box of solenoids that needs a lot of work to program. Is it really better than, say, doing it all electronically via a MIDI piano? Maybe you should ask him before you start building)