It depends on who is he, but he must explain to you what is going to do. Before working on your site you must take a fresh backup and allow him to change the site. If you sign a contract with the support company, you must ask in case of a site failure to pay you money, and of course max time of implementation of changes.
It is good to have a mirror site for testing and after to move the programs to the production site. Of course, it all depends on how big you are and what you pay.