There's so many things, I wouldn't know where to begin... Perhaps the best place to start is to look at the existing studies that have been undertaken and try to produce something that hasn't already been done. Like in psychology, before you begin any research on a subject, you want to see what has already been attempted on it. More specifically if your friend is simply building a usable website and wants to highlight how it is usable (rather than performing unique research like the title of this thread implies), I would go right back to the basics and account for the various usability considerations that exist.
Books worth purchasing (as the case may be) include:
- Don't Make Me Think! - Steve Krug
- Rocket Science Made Easy - Steve Krug
- Designing Web Usability - Jakob Nielsen
- Prioritizing Web Usability - Jakob Nielsen
Those four books will cover all the fundamentals your friend will need. It's also worth checking the PDF files at usability.gov as they have a whole range of good solid guidelines to help in creating a website. And visiting useit.com (the homepage of Jakob Nielsen) will also be worth the effort. (There are other places which cover stuff like Web Accessibility and UX Design which come into the Usability scope which may be worth considering - the best book on Accessibility is called "Web Accessibility" and was published by FriendsOfED). With those four (or five) books, he'll have enough credible sources to back up his compliance (if he follows them) and their not the most expensive titles out there (though I'm sure a library has a few copies somewhere that he can rent out).