Well, that's difficult to say without knowing more about its condition. For example, does it still have its legs?
Of course, even more importantly, are the electronics intact and fully functional? You won't know that without a full set of tubes.
Finding an appropriate speaker is another matter. There are companies that still deal in tube based equipment - mostly guitar amps and vintage organs. The RCA theremin falls into the realm of antique radios.
Unless some other RCA owner wants to sell or donate a set of tubes, your best options are to try to buy them on ebay, but there's often no guarantee the ones you'll get will be any good. One of the tubes is extremely rare and difficult to get.
I can offer to put a set of tubes together for you if you are interested. I have them "in stock" and can test them. We'd have to discuss price offline.
But, to try to answer your question, in beat up or unknown condition it may sell for three to six thousand USD. In fully working condition you "may" expect four to ten thousand. In pristine condition, they can go for even more.
It really all comes down to what someone is willing to pay.
There is an extremely short list of people who can service these instuments if your friend is interested. Can you tell us about where you live?
BTW....Please thank your friend for saving this theremin from the dump for us!!!