Can you lock the sorting order in Mail? (OSX 10.11)
There doesn't appear to be an easy way to do this, so I wrote a Mail plugin for it. You can download it here, and the source code is available here. I wrote it for macOS Sierra, so if it doesn't work on your mom's computer please let me know.

To use it:
1. Open Mail and select the tab you want to lock.
2. Close Mail.
3. Copy the .mailbundle file to ~/Library/Mail/Bundles.
4. Reopen Mail. The header should now reject clicks.
5. To uninstall, remove ~/Library/Mail/Bundles/MailHeaderLock.mailbundle and restart Mail (and to change the tab, move the plugin to somewhere else, and follow these steps again).