If something is missing from Settings, you follow the R's- restart, reset, restore.
1. Restart the phone. I think we all know that one.
2. Settings>General>Reset>Reset All Settings. This resets the settings back to default, but keeps the users apps/data.
3. Backup the phone to iTunes and/or iCloud, then Restore through iTunes and Setup As New. Test the issue. Is it resolved? Restore their backup. If the issue returns from the backup, then there's some corrupted data and they'll need to manually get their data back.