Well, protein in itself doesn't build muscle. Its just a source of nutrient in a process that
allows the body to repair muscle tissue. Its certainly important in any fitness regime and
normal everyday life. The trick is knowing how much protein one needs.
BMQ is not a muscle strengthening course. You don't spend hours at a gym pumping iron
or utilize the body exclusively in that way. You'll be marching, running, moving around,
climbing obstacles, carrying loads, primarily moving from point A to point B as quickly
as one can. The body will adapt by increasing cardio capacity and physical endurance but
you won't leave BMQ with more muscle mass.
The St.Jean mess provides good meals and you can tailor what you eat for better
protein/carb ratio. You need the carbs but theres lots of protein sources provided.
I'd question if protein powder (and its not cheap) in addition to real food would
provide any benefit during this course.
Your friend may have lost weight during the course, but likely it was due to fat/water loss
and the change in physical training. I doubt extra protein would have helped.