Don't delete anything yet.
I prefer to install WordPress in a sub-directory and then "move" it to the root when I have things running. "Moving" from a sub-directory to the root is a copy and edit, move two files and some minor changes in the Dashboard. But they need to be done correctly and in the proper sequence.
Move WordPress:
http://codex.wordpress.org/Moving_WordPress#Moving_WordPress_Within_Your_Site
or
http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory
You can cut and paste from the html pages into WordPress, just be careful of the formatting, I paste in as plain text (an option in the editor) that strips out formatting and then go back and format as needed.
A static site would mean probably all the info on Pages. Also set WordPress to show a static page at the start. (Dashboard, settings, Reading, Static Page)
Also find a Theme your friend likes, some are designed for a static page based site. (http://wordpress.org/extend/themes/)
When the site is up and running then go back and clean up the old code as required.
PS - Rename the index.html file when you move to the root so index.php will execute.