Editing site where theme was changed

I am updating and modifying the design of a website that was built by someone else. This person used the twenty-ten theme but made changes the theme rather than creating a child theme. What are my options for working on this site? Should I:
a) create a child theme for the site?
b) make a copy of the theme and rename it?
c) leave it as is and make my modifications to the existing theme?
*no, this is not a test*
Here is a link to the site: http://www.sherihendricks.com

Thanks!