It is not harmless, especially the more he smokes it anymore than your smoking is harmless to you. If you haven't already told him not to worry about it, I would just use it to help you and him give up the smoking like you want; the money can go for much more important things. Besides, if you are in the U.S., it was outlawed Federally in March of this year, your area many not have gotten around to enforcing it. It has been showed to be much more addictive than regular cannabis, can be like coming off narcotics. Do you two want to be stuck on this stuff all your life?