if he is a beginner, and he obviously is, I am not sure how good it is for him to start with any phone wallet. Many things can go wrong there, just out of ignorance.
I worked as an internet provider reseller, and in general, most "beginners" do not have a clear idea of what online privacy means, or the importance of passwords/private keys.
If we're talking about a small amount of bitcoin, there's really nothing wrong with a mobile wallet. It's mostly for learning purposes in the first place. And there's a reason why I mentioned that OP should help gramps correctly setup the wallet.