My deepest condolences to your friend, Wolf96. Heads would literally roll if I found my koi dead and could trace it. Just reading this makes my blood pressure rise. I actually had what I thought was a close call with something like this. One day when I was sitting out at my pond, I found a knife that I didn't put there. Just the thought that someone strange had come close to my pond with something like a knife made my blood boil. Luckily, I found out it was a friend's who had dropped it the weekend before when he was helping me look for my lost dog.
I hope this doesn't discourage him from the hobby. Do keep us updated if any legal action comes from this.