You can file this complain in Y! as it is considered as cyber crime and fraud. I have answered you before. But, would like to give you more precise answer. This crime comes under threat and not only deactivate the ID of the person but can work practically as a criminal offence. Keep all the messages of threat of him so that if needed IP address can be trap.
This is the physical address of the Y!:
Yahoo! Customer Care
Attn: Mail Abuse
701 First Avenue
Sunnyvale, CA 94089
You can report this. And, file a police complaint regarding this for proper action. File your complaint with proper header information. You can get this as
To learn how to display the full headers in a Yahoo! Mail account, please visit the Yahoo! Mail Help Desk at:
http://help.yahoo.com/help/us/mail/config/config-11.html
You can file this report by mail by visiting : http://help.yahoo.com/
But, don't write to deactivate his Y! ID, but write to track him so that he could be catch and to stop him for further indecency act which ruin your friend image. It should be esclated above then. Believe me what I said to you. I gave you enough information to help you. I am able to do so because I myself working for Yahoo! and come across the cases like you.
Don't delete his any of threatening mails. It will help you much further to track this criminal and don't do anything if he further blackmail your friend. If this person is considered cheap, then your friend also turn cheap and blind for fake online love. Learn to maintain your decency till you not married. Learn from this mistake forever.
Posting your question again and again won't help you much but if you take firm and suitable action will help you a lot.
I don't know, if the temptation is so much in Indian Mind, who forget their own culture and go for this and body enhancing which make only fake identity and beauty and create fake image(exception is there).
God bless you!