Bash the rosehips in a mortar and pestle to break them up slightly and split the fruit. Remove and discard the seeds.
Add the salt to the split rosehips and give them another quick bash with the pestle. The goal is just to break them up a little, not turn them into mush. Scatter the rosehips on a baking sheet and put in the oven. Immediately turn off the heat and leave in the oven until they are dried but not burnt, about 30 minutes.
Add half of the sherbet to 4 1/2 cups warm water. Stir and drink freely.
Keep the remainder completely dry in an airtight container in a cool, dark place for up to 6 months.