Don't worry, it's not too late.
It seems like your friend is not willing to make the commitment to his fish. The nitrate levels in that tank must be very high.
Anyway, the fish can probably still be saved. It will take a lot of work on his part though. Basically, I would recommend an immediate water change of 50%. Hoover (vacuum) the gravel thoroughly. And then, clean the filter thoroughly, and rinse the sponge on the Aquaclear. Do not throw the sponge away yet. Remove the old carbon from the filter and discard, but do not put new carbon in just yet. Do not put food in the tank, unless you're sure the fish are ready to eat. Uneaten food is bad for water quality. The next day, perform another 50% water change. Don't worry too much about the water change, the fish are already under stress and the clean water can only help. The reason for a partial water change, as opposed to a complete change, is to avoid a very drastic change in the water's ph.
After the water changes, add some metronidazole to the tank. Metronidazole is easy to find. Better yet, find a medication that not only contain metronidazole, but also a wide-spectrum anti-bacterial. Follow the course of treatment. Change 25% of the water, add new carbon to the filter. And then feed the fish. If the fish still doesn't eat, remove the carbon and add methylene blue to the water.
I have experienced the same situation such as that years ago. I inherited some fish from the teacher who used to teach in my class room. The students, ofcourse, never bothered to change the water during the ten months that the tank was in the classroom. There were some swordtails, algae eaters, and a couple of small firemouths in the tank. All had the sunken in belly look of fishes that hasn't been feeding for quite some time. But the fish not feeding never stopped the students from dumping food in the tank. I did the water change routine, as outlined above. And used metronidazoel first, and then methylene blue.
The Firemouths started eating within 8-12 hours of adding the metronidazole. Five days later, the others were still not feeding. That's when I added the methylene blue. All of the fishes got back to good health after that. The whole process took a little longer than two weeks. Figure 2-3 days for the water change, five for the metronidazole, and five for the methylene blue.