Good day Dahlia, welcome to the Cove!
Not being a welder myself but having some familiarity of the industry through my NDT work, I want to refer you to the American Welding Society. It has
reference books and
educational materials that I expect would be useful in exams. You can refer your friend to the
AWS certification page. It indicates no particular course is required, but indicates applicants can use their QC7 standard at test time, so I would advise him to become familiar with that at the very least.
I do not have experience with any on-line courses, so I regret to say I cannot make recommendations there. I can, however note the exam includes a practical component: the applicant must have his work inspected at the test facility as per
AWS requirements. (look in the Exams link on the Certified Welder Program page) So his online course and/or book study should be accompanied by hands-on training and/or practice.
I hope this helps!