Email tracing

One of my friend is getting some porn and bad wording emails. Now problem is its ID is like similer as his own ID( if its ABC@yahoo.com then other ID may be ABCD@yahoo.com). I know we can trace that from email header. But its not working. There may be some mistake from my side to interpret them. But as per me I am sure that I have interpreted them almost 99% right.

Is there anyway to traceout that email person’s PC….? And is there any authorised site where we can complain this Cyber Crime sort of thing…?

Thank you in advance.

Acceptable and Non acceptalbe answers are welcomed.