The most important thing is "having an idea of 'what to do'" and he should have that.
One time (circa 1967) Roscoe, Lester, Joseph, Malachi and (maybe) Phillip Wilson were playing at an informal session at the University of Chicago. Joseph moved to piano (for a short time) and did "all the right things" to add to the texture and move the music along. I'll never forget it.
Not really of "importance" overall, but nice to have in the mix.