Having worked with claris a bit myself, I can safely tell you it is
unlikely the keys are brass.
If it is very old, the keys will likely be "potmetal". Nasty stuff
to work with.
If it is more recent, the keys are likely to be nickel.
Either way, replating can be done relatively easily (though I would
get it done prefessionally). The tricky bit is the plating will
change the dimensions of they key, this may cause problems. If your
friend is a music professor though, I assume that he would know this
and would be prepared. Who knows, he may even be correct on the
brass keys - although I doubt it.