If you are an Administrator, try adding his mailbox to your Outlook Client. Launch Outlook, Tools, EMail Accounts, View or Change, More Settings, Advanced, Mailboxes, Add.
Then just add his box as an additional to yours. Then back in outlook you should be able to see his box along side of yours.
This way you will be able to see the users sent folder. Other than that you would have to go through the database to find the old messages.
Do you have message tracking enabled? Going forward that would be a better idea but it will not help with old messages before it was enabled.
Another way of doing this is to enable message tracking and logging on your Exchange server. This creates a log of every message going through the message transfer agent on the server.
In addition to allowing you to track and log every message sent and received by all users on the Exchange site, it is very useful for troubleshooting various situations where you might need a detailed log of message activity.
To see the messages, you can manually open the log, or you can use the Exchange System Manager tool and go to Tools/Message Tracking.
To enable message tracking, right click on the server name in Exchange System Manager, go to Properties.
On the General tab, you’ll see a check boxes to Enable subject logging and display and Enable message tracking; you will also want to set a maintenance period for removing log files so that they don’t eat up your drive space – I use 10 days, but you could select any time period you want.