I have no idea, but then why would you need to know?
I know; you are putting together a business plan. Well, if you were to get real and valid numbers it would look bad. The market isn't that large and is pretty well covered by the current manufacturers. If your friend has something new and innovative he should just start making them and selling them in a local shop. If they catch on he can sell to other shops and expand from there.
Trying to put together a business plan for a small market like this is looking for an excuse to not do it.