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In an earlier reply I credited your explanation on this issue, maybe to missed it.
I had a friend in JROTC who was in very good shape, much better than I was. I went on to ROTC in college but he was denied entry into the military because he had a history of asthma. It sucked for him but it makes perfect sense that in a wartime situation, an asthma attack could really screw an operation up.
So, in that instance, your friend was denied entry into the military based on an existing health problem.
But you also said he should not be denied as long as he can do the job effectively.
Unless you allow him the chance to do the job, you'll never know for sure if he can or can not do the job.
Your comments are in conflict with each other. You can't do both -- deny a recruit based on an existing problem AND allow them to join to see if they can do the job.
Pick one.
This is why waivers exist. If your friend were going into a support field such as computers and communications that doesn't require perfect health, he could request a waiver. The service would do what they need to evaluate him using additional tests, documented history of sports or other physically demanding activities, and whether the ailment has increased or decreased over time (asthma often disappears after adulthood).
I was rejected due to a medical standard, but i received a waiver which allowed me to complete ROTC and be commissioned.
Again, experience demonstrates how the system actually works, not based on thought experiments, beliefs and assumptions.