I am a guer, and how is logic I have non jewish family. They decided to get closer to Torah by acting as benei noah. They don’t want to become jewish, but they always ask me things. How much can I teach them? Can they fast for 17 tammuz and tesha be av. ? Can they learn about the 3 weeks? How much to teach them about holidays?
They may certainly be taught anything pertaining to the Seven Noahide Laws in detail. Aside from that you can answer their general questions about Jewish practice and holidays. There is no prohibition for them to fast, but it should be explained to them that in the Torah view this is not what is required of them and their focus should be on keeping the Noahide laws.