Work Email with Personal Details - Ethical Challenge

The case scenario:

You are the network administrator for a rather large company. As part of your responsibility as a network administrator is to monitor the emails for the organization. Usually this just means occasionally allow through emails for staff members that have been accidentally blocked by the spam filters. You get a helpdesk request from a staff member asking for an email to get released. Normally it's standard procedure except this time the request has come from the wife of a very good friend of yours. You recognize the name on the helpdesk request so quickly attend to the problem. As part of the procedure you need to manually open up the email to ensure that it isn't spam, so you do and you discover that it certainly isn't spam. You find that it's actually an email to your friend's wife from her lover. You scan the rest of the contents of the email and there is no doubt that she has been having an affair for some time now.

You release the email, but you can't decide what to do. You're initial reaction is to call your friend up and tell him about the email, however you quickly realize that company policy is very strict about revealing the contents of confidential emails of staff members regardless of the contents and unless someone's life is in immediate danger, under no circumstances are you permitted to reveal the information. In any case you know that revealing this information presents great risk, because even if you don't do it directly, there is a good chance that the dots will be joined somewhere along the line and you will be found out. However you feel that by not telling you friend that you are aiding his wife get away with adultery and this troubles you greatly.

The solution addresses what should be done and why; citing the company policy on managing emails.