Good advice. I have had my wife monitor the messages today but nothing new has come through.
The only downside to doing this is that there's a chance that just as we can see their iMessages come through on my wife's handset, they could receive any iMessages sent to my wife too. (please correct me if I'm mistaken). Therefore, we can either a) turn iMessage off on her phone (resulting in her not getting their iMessages) or b) Change her email address associated to her apple ID per Apple's instructions (resulting in her not getting their iMessages).
In other words -- in order to continue to monitor their possible conversations, we may run the risk of them seeing her conversations as well.
go to the message coming in from the person, click on "Contact" then you can see what email or number the message is coming from if its associated with your wife's email ID or not...the number/email the message is coming from will be blue.