All I could come up with (this fast
) is the following:
Add another .pst file, and pass protect it. Creat a rule to move every send message (or a copy of it) to a folder in that added pst file.
The thing is Outlook only ask for the password once in an Outlook session; when first using that folder. This means that when your mother doesn't close Outlook, the messages in that folder can stilll be deleted.
But, if she closes her Outlook, the only way to view or delete the messages is if the person has that password.
I don't know the exact situation, so I don't know if this will work for you . . .