Better for your friend to find out before they got married. As hurtful as it is for your friend, you have to admire her honesty.
I would suggest that your friend breaks off the engagement and relationship, until the �net guy� issue is resolved with this girl there will be no room for your friend in the relationship.
On the other hand I could just be an excuse to break the engagement and there really is no cyberman. Either way, she is telling your friend something about their relationship and he needs to get the message and act in his best interests. If he doesn�t then he really is a dope.