Welcome to the forum!
First, let me give you Kudos for getting the Air Force veteran his Purple Heart and wish you good luck on the other one.
I am not familiar with regulations on the requirements for earning the Purple Heart from the Army nor the Air Force.
I am quite familiar with the regulations on the Purple Heart’s requirements from the United States Marine Corps, or at least those while I was serving. They may have changed since I retired.
The basic rule was that to be awarded the Purple Heart while a Marine was that the “wound” must have come from a result of enemy action and fire. “Injuries, diseases, immersion foot, jungle rot, etc., were not considered due to enemy action or fire, therefore not eligible for the Purple Heart. You could break an arm or leg, have them rot away, or suffer malaria or many other types and you would not be eligible for the Purple Heart. Trust me, I KNOW the Malaria thing.
So, I find it interesting that the Air Force would award the Purple Heart for frostbite. But, it may seem that the Army has basically the same restrictions as the Corps does, I guess.
I hope this might address your questions a little. It might not be that the Army is trying to be “hardass” on this, but that their requirements are different from the requirements for the Air Force. I know there are medals which are awarded by different branches but have different requirements depending which branch the member served in.
I hope this helps.
Good luck on the second medal! I hope you succeed.