Do more water changes might be helpful. Generally some positive response is found after water changing.
Water temp matching and dechlorinating.
Do you have Prime water conditioner there? It helps to neutralize the toxic things, ammonia, nitrite and nitrate. Dose as directed.
Maybe water changes were not done.
Or too much food was fed possible, causing unsafe water quality.
Or filter pads were changed /thrown out and you lost the beneficial bacteria?
I'm so sorry you are far away and can't address this directly.
Getting the water tested at a local fish store might be helpful. Find the tubes. A lfs
might have extra tubes and be willing to give or sell them to your family member.
Knowing what you are dealing with is very important.
It's likely something was a cause. Water quality or bullying from another fish, or an illness.
Sometimes a stress in the tank can cause illness.