Definitely a real piano...not an electronic keyboard.
I HATE Yamahas. They have funny action, tinny sound, and the octave is actually a little larger than a Steinway (measure and compare, it's weird).
It should be a piano that the student is comfortable playing--one that the student can control. If he/she can play loud, soft, fast, and slow (and all combinations) with a controlled sound and in a relaxed manner, this is a good first step.
Then, just use your ears to decide. The brand isn't the end-all...as each piano is unique (even the same make/model pianos can sound drastically different).