Well Google's Gmail user interface, including most other email services, is pretty basic and simple to pick on by just using it. From the sound of it though, you cousin is apparently new to using a smartphone from an overall matter. If that's the case, I'm going to make a supposition your cousin wouldn't pick up too much from reading online help guides. There's a disconnect in our heads with performing daunting tasks while at the same time reading equally daunting articles.
I'd suggest your cousin take a basic course on smartphone usage, a setting where there's one-to-one, hands-on help when necessary. Where does your cousin reside? There are local community colleges, libraries, community centers, etc.that offer these kinds of 'primer' courses.