Ok, have him try this and see if he can get back into his Yahoo! email account.
1. Go to
http://www.yahoo.com/
2. Top right under the search box is a link for signing in, click it.
3. On the right is a sign in box. Under that there is a link "I can't access my account", click it.
4. You are taken to a wizard to help you recover your password info.
You are going to have to solve this problem first as you stated that in the WHOIS records this is the contact email for the domain.