How long has the tank been up and running with fish in it?
How many gallons/litres of water does the tank hold?
Have you been doing any partial or whole water changes?
Thank you for the information.
The most important symptom you described was that the fish has black spots.
Black spots are an indication of something called ammonia burn and occurs when a fish has been exposed to ammonia in the water.
Ammonia is harmful and often lethal to fish and should not be present in an established aquarium. It is caused by fish waste and decaying matter in a fish tank and in an established fish tank there generally should be enough "good bacteria" in the water to break it down thus rendering it inert.
Another pertinent fact you shared was that a neighbor was taking care of the fish and many a time this turns out problematic as many people who are not fishkeepers tend to overfeed fish while under their care thereby supplying the fuel for an ammonia spike.
A drastic change in water parameters such as water temperature, ph, etc.,etc., as a result of a water change can also cause a fish to become ill.
Usually the first thing I would recommend in a case like this is that 25%-50% of the water be changed but as you've already done that yesterday we will bypass that recommendation.
So the first thing I'd suggest is to make sure the water temperature is stable and at a level suitable to goldfish which is a range of 16c-22c degrees. After establishing a stable water temperature add some aquarium salt to the water at a ratio of 1 tablespoon per every 8 litres of water which in your case is about 3 tablespoons.
After the temperature has been adjusted and the aquarium salt added begin to treat the tank/fish with a medicine called Interpet Aquarium 5-General Tonic which should touch upon any and all issues the fish has, click on link below for a view of the product.
Keep in mind that the fish's chances of recovery are heavily dependent on how badly it is being affected and for how long it has been affected. The fish's overall make-up and genetics also come into play with regards XXXXX XXXXX chances for a full recovery.
I don't believe the other fish will encounter any problems as this appears to be more of a water quality issue rather than a disease issue and the other fish has been able to cope with it. Once again a fish's overall make-up and genetics comes into play here.
So what I recommend be done is more frequent water changes, perhaps every week, making sure to use water that is the same temperature as the water that will be discarded.
Also in the future if you leave the fish to be cared for by someone else it would be important to stress the point that the feeding of the fish be light.
Best wishes on a speedy and full recovery for your goldfish.
Please let me know if you have any questions.