OK,

Today my friend's mailbox received hundreds or even thousands un-delivered emails.
Someone from IP addresses 216.3.xxx.xx (Digex.net block) using many open relay sendmail machines sent thousands spam emails with his email addresses in the from field.

He just sent complain message to digex/intermedia hostmaster/abuse departement.

In return he receive tons of un-delivered emails, his server hang for hours and will receive many complains since this pathethic b*stard spammer offer amazing money opportunity and put a note if the receiver would like to be removed or interested with the program simply reply that emails.

What I believe that this person tried to distroy my friend's company's reputation and make http://mail-abuse.org/rss/ block his servers.

Now I need your advice, what more he can do to solve this problem?
He put his servers into Above.net NOC.
Should he call above.net for some help to contact Digex.net?

Spammers sometime send a bulk mail out with a Return address of another company, so does not go back to the place of where the mail was sent from.

This could be the case of Mail Termination.