My family friend have a fish tank of 5 gallons which has 4 goldfishes and few more different types. Every 2-3 weeks the tank was cleaned. She noticed that one of the goldfish turned white from fins and since she was unaware of it, it increased till tail. She added parasite treatment but it didn't work. Pinches of salt were added but no improvement was seen. It increased badly so she separated from others but unfornately the remaining goldfishes caught it too. Fish care specialist(white spot) is used but its not helping. 2 of them have died and she wants to save the others. What should she do now.

First you have to identify what is really going on before treatment can begin. Some white spot (ich) problems can be treated in specific ways. But without proper identification you can treat for something that doesn't exist.
Cause, Treatment, and Prevention of Ich in Freshwater Fish

First and foremost any infected fish should be isolated and maybe taken to a fish store near you to identify the specific problem.

A home remedy for a lot of different ailments when you aren't quite sure of the specific cause is a small amount of salt added to the water.

Yes, they are freshwater fish, but a small amount of salt for a few days is enough to kill parasites and help heal the fins. You will need to do several water changes afterwards to rid the tank of salt though.

Here is a great link about using salt in goldfish tanks, you need to do more than just add a few pinches of salt: GoldFish Emergency 911 - Salt Treatment for Goldfish

First and foremost, a five gallon tank is WAY too small for even ONE goldfish... never mind four, plus some other fish. That many fish in a tiny tank will cause a constant build up of ammonia and other bodily wastes (toxins), which stresses the fish terribly. Even a daily water change will not be enough, and that many water changes is a source of stress all by itself! Stress and toxins kill fish, AND can cause a wide range of physical changes, including color change before death.

When you say that the tank is cleaned every two to three weeks, I suspect that you mean that the tank is stripped clean? This is not the way to maintain a tank, even a larger one. It is hard on the fish and hard on the person who has to do it, again and again. Over cleaning a tank removes the beneficial bacteria that builds up in the gravel and the filter media. This bacteria is crucial in a tank to control the build up of toxins. It would be very beneficial to your friends (and their fish) to get a VERY solid knowledge of the "nitrogen cycle", and learn about better and kinder methods of aquarium maintenance. Check out aquariumadvice.com and do some reading. Everyone will benefit!

The best way to treatment of the fish is to find out the correct reason for the problem. Once the problem is identified correctly Then start the treatment.