Trying to identify a fantasy novel for a friend. Not sure of publication date, but she picked it up at a library sale in the mid-90s.
The protagonist is the Prince of an island nation, and there's a coup spurred by an evil sorceress with mind control and illusion powers. They are saved in part by the discovery that a kind of berry that they thought was poisonous was, in fact, only poisonous when not diluted, and in fact gives resistance to the sorceress' powers.
At one point in the story, a wizard is forced to swallow one of the berries, and it supercharges his magic, and as he walks through snow, his footprints become snowballs because he has so little control of the situation.
It was written in English, about novel length. It may have been the first in a series, at the very least it ended with a possibility of a sequel.
Any help in pinning it down would be appreciated.