It is awfully far north for courgettes, which may be a factor. Maybe they need a good year as well as good soil to thrive. And the actual temperatures and sunshine hours may matter as much as the soil.
However, we do not know the ph of the soil and it could matter, the variety of courgette or the actual weather. Does Gothenburg generally grow courgettes or are they really difficult and marginal. If they are a marginal crop that far north, then maybe planting through black plastic on top of the mulch would help, because it warms up the soil. Like plotstoeat suggested. As of course would a cloche cover in the early days before the plants are fully established.
The good initial year might have been a weather fluke and the decreasing yield may be due to factors outside of the soil.
I would measure the ph value just to make sure that it is not way off, then I would add soil warming (black plastic) and protection (cloches).
Not knowing the growing conditions in Gothenburg, it would help to know whether all gardeners find courgettes difficult, because they are right at the very edge of what can be grown, or whether courgettes grow quite well and it is only your friend who somehow has a problem that others do not have.
I remember a gardener from Orkney on this forum many years ago, who could not grow courgettes even with hand pollination in a greenhouse. It sounds to me like there might be a geographical challenge as well as a soil challenge.