If I understand the OP, this was in Germany. Here in the USA, most states (maybe all states?) have what is called "the Good Sameritan Law". The Good Sameritan Law says that if you come to someones aid in good faith and according to your ability, you cannot be sued.
One observation here. Keying in on the "according to your ability" part of that law. I totally agree with the good samaritan law, For instance if you are trapped in a burning car, no reasonable expectation of help before you burn to death and I pull you out of the car to save your life you cannot sue me for being paralysed because as a lay person I only intended to save your life, Not offer you a full medical evaluation.
On the other hand if an EMT came along and the car wasn't at risk of burning and he jerked you out of the car without placing you in a c-spine collar guess what? Yeah, Hes in an ass load of trouble, and should be. Where the water gets muddy is this... What if the car is burning, and the EMT is afraid it will explode killing both you and him? Can he make the judgement call? I don't know.
What I do know is this will be the arguement, The instructor is not an ordinary diver, He is held to a higher standard, " according to
HIS ability "
My feeling though from my heart is this, He (the instructor) is still a human, and we as humans make mistakes, He did intend to help the man, and the diver knew (or at least should have) the risks before he got in the water, he went of his own free will. All that said I believe the instructor made at the time his best effort, right or wrong to aid the diver in trouble.
I can only hope his lawyer demands and gets a true jury of his peers (Divers) to judge him. This is a very complicated situation. Imagine trying to explain all the ins and outs of scuba to 12 random people off the street? What a nightmare.
My thought and prayers are with both the diver who passed away, the instructor, and both their families who will be forever touched by this tragic event.