Although it's very sad, I think we've all been through that experience to a lesser or greater degree, even the loss of one fish is a real blow.
Over the years I've tried not to get too attached to any of my fish, though it's difficult.
I'm long past the time of giving them individual names and I always try to find a demerit, even a small one with any fish, which I will use to "lesson the blow" if it ever turns fins up.
Those unconnected with koi assume it's a very "relaxing hobby" when at times we know it's nothing of the sort and can cause a lot of personal stress if we don't try to control our emotions.
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"I don't mind if you don't like my manners!
I don't like them myself, they're pretty bad,
I grieve over them on long winter evenings." | 
Absolutely gutting and sad to hear! I hope your friend can get over it without loosing heart for the hobby!
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