If you set yourself up as the kind of admin who redirects mail, you have to redirect it. There's nothing illegal going on, and flagging it up as a violation of company policy on computer use is obviously messy and unnecessarily complicated. The right answer is obviously to stop nannying your users and let their emails vanish into the void. Teach them how to look up email addresses and check their sent items and then just leave it the hell alone.
The implied moral quandary over being complicit in an affair between two adults is so absurdly puritanical, I can't help but wonder if the admin worked in some kind of hyper-zealous bible sales business.