It's probably a mix off all of t he above listed.
He just got this tank last week IIRC. Chances are the ammonia levels are close to their highest point right now, so I am not surprised he lost something.
New fish die off often, that is why many in the hobby keep QT tanks that are well established so ensure that a mini cycle doesn't kill them off.
I suggest you check your water. If it checks out fine, most fish stores will let you bring the fish back.
Keep a close eye on your water parameters for the next few weeks. If you do not keep a close eye on the parameters you are likely to loose more fish.
It is also recommended to more the filter pad from the old tank to the new tank, assuming you haven't thrown it out and it is still wet. I would still throw it in even if it dried out.
DO NOT add any more fish until after your tank has finished cycling.
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