I did something like that, but for good reason. I took 6/8 class periods, but that included college writing, AP calculus and Honors Physics (we didn't have AP), which finished off all the required and recommended classes for admissions. This meant that I didn't get to school till 0915 one day, and left at 1230 the other (block schedule). On my late mornings, I went running and did strength training so that I would have more on-court time for tennis in the afternoon. On my release days, I just went to the courts early to work on serves and get some extra hitting time to work on fundamentals on the wall or ball machine. This gave me a solid 3hrs/day minimum on the courts, without giving up my study/CAP/music time in the evening. I was pursuing a tennis scholarship at the time so for me, tennis was almost as important as school, but I knew calculus was going to kick my butt and that I wanted to learn it well, not just get the grade.
The point is, if your friend is going to take a light schedule, there better be a good reason beyond just having an easier year. IMHO, there are a lot of things that will be better for you than filling your schedule with classes you don't need, but make sure you aren't just meeting the minimum requirements. If AP calculus, an AP science, and an AP English (or equivalent) courses are offered, and you haven't taken them, that's a problem. If you are choosing something like biology, anatomy, creative writing, etc., a free period to focus on the other classes or something else wouldn't be a bad idea.