Swollen Harlequin Rasbora - Is It Dying?

First to say your friend is very old school. Fish do not die from lots of clean water - discus breeders do daily 50% water changes! It's not a question of the fish getting used to the water - if you do regular changes the water does not change very much as it doesn't have time. Your friend's tank water will be quite different from her tap water and if she does do a water change, she'll change the water chemistry so much at once that it could easily kill her fish. And if she got new fish they may well not be able to cope with the change from the shop to her tank. Although, if her tank is very, very heavily planted, she would be OK - google the Walstad method.

How long have you had the harlies? Is it just them that are showing these symptoms? If it was an infection/parasites I would have expected other fish to show signs if you've had them more than a couple of weeks. Do their scales stick out when they get fat?
eSHa 2000 is just about the best medication we can buy in the UK. I would be inclined to use a course of treatment for the harlies - do you have a quarantine tank as it would be better to treat just the harlies if possible. If not, you'll have to treat the whole tank.
But if their scales stick out like a pine cone, that's dropsy. This is a symptom of several diseases rather than a disease itself. The problem is working out the underlying disease to know how to treat it. However, the scales sticking out is a result of kidney failure and the resulting fluid build up, and once the underlying disease has reached this stage it is usually too late for the fish to recover.

Fish can look like they're about to explode when they've over-eaten.

As a general rule of thumb, a fish's stomach is about the same size as its eye. I know it's not really possible to work out, but ideally the amount of food should be equal to one eye per fish.
And underfeeding is safer than overfeeding.