Admins have a lot of priviledges. If an admin needs to see the conversations of one user to another user, there are a couple of options.

Create a Superuser Password and log in as another user in the domain.
When logged in as another user, view their IM’s with other members by choosing a member from their contact list to open the IM window.
Export the domain data as XML and inspect communications by searching for keywords.
From the XML, you may be able to search for the user’s ID first, then read through the content belonging to that user to discover the information you, as an admin, are looking for.