How to block any app from being installed?
First, disable the primary source of installation. On most devices, it is the Google Play Store but your device might have been shipped with a non-Google market app as well. So go to Settings &rarr; Apps &rarr; (three-dots line &rarr; Show System) or (All Apps) &rarr; your market app &rarr; Disable. 

Second, disable the Package Installer system app. This provides the GUI for sideloading. So if you disable it and attempt to install an APK (through a file manager, by drive-by download, or when pushed by a third-party app) you would definitely fail. At this point, it is irrelevant whether Unknown Sources is disabled or not, but you may keep the latter disabled from Settings &rarr; Security. 

Third, an app can still be sideloaded through command-line, preferably through ADB. Keep Developer options disabled and hidden (default). This is untested but revoking executable permissions (requires root access) from pm binary for user 2000 or for every user might help. This however, may cause device instability so take backup from a custom recovery before you proceed with this step. 

Your user would be fine with the first two steps. It's not like they would deliberately try to find a way to install an app if they are not tech-savvy already. 



Alternatively, in your particular situation you can use an ad-blocker app. The best ones require root access though. 

Or, why not use a kiosk-mode app? Limit your mother's activities to certain set of apps she should use. That's an another option on the table. 

Choose wisely.