b0untypure1: that's the major flaw in Xero accounting, lack of security and ownership transfer. Also there aren't detailed logs so he could very well be changing things, making payments, without you knowing. Im pretty sure it only logs the log in user and time of log in, not the actual activities carried out.
There is in fact a fairly recent audit trail feature available to users on the "Adviser" level.
If you look under the Adviser menu you'll find "History & Notes Activity".
While it doesn't record everything it provides a pretty good overall insight into the activity of users.
Here's the help page: