The Gmail app for Android doesn't support logging out of a Google;Gmail account like the Yahoo Mail app does. The only way you can log out of a Gmail account from the Gmail app is to delete/remove the Google Account from the device
through Settings > Accounts. The Gmail App is just one of many apps that can access or use/require the Google Accounts on an Android device. Several other apps, including non-Google apps, can access those accounts if they acquire the necessary permissions.
A Google Account and associated services are a central part of Android. Of course you could get rid of all Google Accounts on an Android device, but that will leave it heavily restricted in terms of features, functionality and security. You need at least one Google Account (main/primary) that is NEVER removed from the device for full functionality of the android device, and any number of additional (secondary) accounts as desired.
Instead of seeking to secure his Google Account on the Gmail app, he should secure his Google Account overall and his Samsung Tab 4. Security ought to start from the device-level (low-level). He can further secure his Google Account by enabling 2-factor authentication (requires more than just a password), for example, a security code texted to his phone via SMS in addition to the password, to log into his Google Account. To secure his account, he should go to https://security.google.com/settings...ogle.com/intro
and start the security check-up process that will take him through steps in making his acoount more secure.