The most you can do is look at the mail headers to see what SMTP server delivered the mail to your server -- but that doesn't necessarily mean much unless the sender's email domain has an SPF defined. I think the easiest thing to do here is to pick up the phone and give your friend a call.

Also looking at the mail headers depends on the email client and to some degree the email server. You would need to give more information for someone to provide you with an answer on that.

Hi sknake/folks
thanks for your reply.
this is a vital case;I'm trying to help a friend of mine.he is really in trouble and have to prove in court that somebody sent some threatening emails to his girl friend on his behalf otherwise he will be imprisoned.
we have the emails which is received by his Gf,and now I am looking for a way to find the Ip Address of sender.
how we can look at the mail header?!
any clue or help is appreciated
cheers

Isnt this what private investigators are for?

we want to do it by ourself, investigators are better go to hell,they are just wasting time.
I am grateful if anybody could help
cheers

The only way I could think would be writing style. Also check time and dates, he might be on a camera at some grocery store at the time of sending. Proving he didn't send it.

You might also have to call the company that is providing his email service and request information on his account. Stuff like log in times and so on. If your lucky they might release the IP's. But the IP would almost be useless due to dynamic addressing and encoding for packet transport. What you would need is the MAC address of the sender.