Do your job
Depending on company e-mail usage policy (which in any sensible organisation would specify that company resources are subject to possible scrutiny, and also dictate that communications should be within certain bounds - including, but not limited to obscenity, illegality, etc.) as a mail admin, people should assume that the mail admins have legitimate access to all incoming and outgoing mail anyway. The option I would go for is noticeably absent from the poll - a quiet word in the sender's ear to knock it off and/or take it out of the company system, because otherwise sooner or later they're going to get burnt. Now, if company policy DOESN'T expressly allow for scrutiny, then your boy has a problem, because no matter the intent, intercepting e-mail is a no-no, and without permission he shouldn't be doing it.
What puzzles me is how internal mail users could get the address wrong in the first place. Don't they use some kind of address book or company directory?