There are a couple easier ways to do outside the host file that would still be highly effective.
If the domain yahoo.com was added IE's restricted site list or to the machines software firewall if it has one it would very quickly make most yahoo functions, especially things like the mail system that relies on cookies and/or ActiveX controls, completely nonfunctional.
However, if all he did was modify her bookmark you might just want to have her remember "mail.yahoo.com" and it will take her directly to the Yahoo! mail system without any other pages to go through, it loads right to the logon page.
Good luck!