A hard one, indeed.... I have tried but can't come up with anything that might be remotely helpfull.
I have one "similar" case in my files when a pvt was accidentally shot by his buddy when Germans plundered the barn where stores were kept and which they were gauarding. The Americans opened fire but shot one of their own.
Although accidental, the man who shot was court-marshalled, I believe.
Also read an article on line last year in which a vet confessed to his family after 60 years that he accidnetally shot a buddy during an attack. Can you imagine something like that and carrying that with you your whole life?
I am sorry this is in no way helpful. I hope you and your family will be able to find out what happened and find closure somehow.