Also, is there a way to convert a directly edited theme to a child theme?

Also, is there a way to convert a directly edited theme to a child theme?

You would need to find out if he changed any of the original theme files. You could make a backup of the current theme, re-install the current version of the theme, create a child theme and check to see if everything is working as it should. You might need to dig through some code to find any customizations but it has been my experience that sometimes people refer to themselves as “wp Developers” and they really didn’t do much developing, just setting up WordPress, etc.

How would I go about this? (I want to edit the copyright and “powered by” text).

I would start by looking for a file called footer.php. You can do this via FTP or via the dashboard. IT is probably in the root folder of your theme.