A friends pond and his annual problems...

i have a lot more questions:

is there other filtration besides UV?
test for ammonia, nitrite, nitrate?
any water changes for the pond, or a stream that feeds fresh water into it?
how old are the goldfish?
what does he feed them? how much/how often?
if he buys a big bag of food at the beginning of the season, possible that it spoils before he uses it all?
any trees or bushes nearby that drop leaves or fruits into the pond? fruits could cause digestive trouble, leaves could lower pH.
do they just get fat or do they have pineconed scales like dropsy?

This could be a load of crap that I heard once, but worth it to find out i guess.... I heard once that sudden water temperature changes can shock a fish and shut down its kidneys which would cause the symptoms of dropsy. Whats the weather like in Scotland, and is he using a heater in the winter?

Sounds like dropsy and I've noticed that overfeeding can either contribute or even possibly cause it.

Just a thought but does this always happen in the fall once the temps cool? If so, it might simply be a case where the fishes metabolism slows due to the cool temps yet your friend continues to feed at the same rate as summer?