Unless he uses something like exchange server, he will be unable to do this.
Early versions of Office, I think 2000 and earlier, allowed network sharing but the later versions do not.
Your friend does have a number of options though. There are freeware and retail products that can do what he wants.
Another option is to have a second mail box, many ISP's allow you to have more than one mail box. You configure the second PC with a sub-address (a different e-mail address to your primary e-mail address). Then you configure your account with your ISP to forward (copy) all incomming mail from the primary address to the sub-address so that all mail exists on both PC's.