I'm sort of torn on this one. Yes, he was an adult, and should have known better, however, he was also young, there is no indication that he knew the effects of radical dieting, and if the recruiter led him to believe that the Army had supplied him with medical or dietary training, then it's on the recruiter.
For example, if the recruiter told him, "Look, you need to go on a diet, maybe even a drastic one..." that's still the young man's responsibility to consult with a doctor.
But if the recruiter said, "Look, they teach all of us how to make the weight requirement, and personally, you need to take some drastic measures like making yourself sweat more, eating very little and possibly even considering throwing up anything high calorie to make the weight. It's perfectly safe, a lot of us had to do it to make weight." then that's the recruiter.