It’s a fish. Fish die. Fish get flushed away. Anxiety?
In that case pretend they never left the fish with you.
That is one of the other reasons I say don’t buy a fish. They need to know there are consequences for tagging a long, before they stow away in the landing gear of a jet headed to Hawaii to stay with CairoCarol.
I vote for replacing Arnie, but not with a beta. See if the kid notices.
OK, so how does one go about giving a fish a Viking Funeral?
When I was a child–well probably a teen or at minimum a pre-teen, we left a pair of goldfish with our neighbors when we went on vacation.
When we came back, Mrs. Neighbor handed us a bag of mail, a bag of newspapers, and said “We’ll bring the fish back later”.
Needless to say, this aroused our curiousity.
Her four year old had hugged our goldfish. The goldfish did not survive.
I’m not sure that actually replacing the goldfish would have been a high priority on anyone’s part, but we let the neighbors get us two new goldfish, and we kept them for the rest of their natural lives. It made the neighbors feel better about letting the four-year-old near the goldfish.
So, I’d also vote for consult the parents, offer to replace the fish, but you don’t need to replace the fish before the owners return home.
Got any magnesium lying around? What else burns under water? 
I was thinking more like freeze it in a large ice cube.
I knew some arty types once who, when one of their aquarium fish would die, would immediately chuck it in the freezer, so they could cast it in epoxy.
that’s an Eskimo funeral. 
It’s a quiescently frozen treat.
The word quiescently means in a restful state. It doesn’t get more restful than that.
So I was backing out of my garage and the SO was off to the side and I heard, “crunch” and thought I backed over a plastic cup. I looked at SO’s face and he looked down and I could tell it wasn’t a plastic cup.
Our neighbor’s backyard turtle somehow managed to escape its back yard and go for a very slow walk through the neighborhood. The tour ended badly.
I called the neighbor and she was more than slightly distraught, but decided against a funeral for her kids and told me to dispose of the remains using Silver State Disposal in a plastic bag.
She bought a new turtle and, less than a week later, they have never seen that turtle again. It may or may not still be in their backyard, but the point of the story is - tell the parents and let them decide what to do.
As far as I was concerned, it was a horrible accident, but not my fault. The same goes for you.
I checked with my neighborhood expert on these things, and he said that fish which die of disease do not make good sushi.
He did say the others ones could be better, but not very filling.
Oh well, that’s what you get for asking the samarai sushi chief.
Flambee it with brandy and serve it with a nice fruity merlot.