I have more than one email address associated with my Apple account. Within the iMessage app settings, I can select which email address(es) can be used to reach

I have more than one email address associated with my Apple account. Within the iMessage app settings, I can select which email address(es) can be used to reach me over iMessage. I guess (but have not verified) that OP's grandmother could solve her immediate problem by adding a second (secret) email address to her Apple account, and having that as the only address which receives iMessages. (Not saying this is easy.)