I don't exactly remember how I triggered Florin's execution but I remember following Erin around in Darchlight caves, telling her Flora was apart of Bladenstar, exposing Flora as soon as we got out of the cave, and telling Cassandra everything.
While Flora is on trial her, Bladestar members bust in the court room and save Flora. Flora had planted evidence, beforehand, to make it seem like Florin is apart of Bladestar and gets him put on execution.