If you want to log all MySQL (just a guess) queries you can go with the solution in this post: Log all queries in mysql
But it should be mentioned that this is not wise for a production site.
Besides, if you really expect that he could have altered the installation with malicious code you should either reinstall a fresh wordpress and transfer all data or at least validate the code files against a fresh wordpress installation to check for altered code.
Basically this is the same as for a potentially penetrated system, restart at a backup point that you trust or with a fresh copy.