Make sure before committing to a theremin teacher that you have heard the person play and that you are certain that you want to sound like that particular musician.
Unlike conventional instruments, there is no established, traditional technique with the theremin. Most thereminists are self-taught, which is something you probably could not say of the majority of teachers of the keyboard, violin, etc. Simply because someone represents him or her self as a teacher of theremin, and is willing to take your $$$ for lessons, does not mean that individual is qualified.
So who is to say whether someone IS or IS NOT a decent theremin teacher?
You are!
That is why, as an adult learner, you should make sure you have heard and/or seen your potential teacher play before making a decision. If you were learning the French horn, or the oboe, or the cello, and you inquired at a local conservatory or school of music, you could be fairly certain that the teachers they recommend had the credentials to give you what you want. Not so with the theremin.
The real question here is not whether the teacher can teach, but whether you want to learn what that teacher has to offer. You are the judge of what you want to learn. As the great theremin virtuoso Clara Rockmore said, "The most important thing, before you start studying the theremin, is to know where you want to go."
Don't tell yourself, "Oh, but I just want to know the basics. Anyone ought to be able to show me that."
That may seem reasonable but it is not true. It is possible to start off on the wrong foot and not even know it until you hit an invisible brick wall and realize that the way you are playing will not allow you to progress any further in the direction in which you want to go,
I know, because that's what I did.
I had to go back to square one and start all over again.
In my opinion, you should not even take any advice on theremin playing from anyone whose playing you have not heard or do not like.