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.
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.