Employed outsourced linux administrator to manage a CentOS VPS (Virtual Private Server), wish to keep track of what he is doing.

From command history, I know he will type some commands, edit some config files, create and delete some files, etc.
I hope to monitor what has been done exactly (e.g. which lines has been change in particular configuration file). Any advise?

Note: I have tried to enable "audit" (introduced in kernel 2.6.x) related functions. They are not working in my VPS. Also, I do not have access right to hardware node...

UHH. Make that part of the contract. It is possible for him to delete any history of actions.

You can insist on him using a version ctrl mechanism, so you've got all the prev versions avail.
Ultimately though, a root admin can change anything, so you're going to have to trust him somewhat or do it yourself.