Sorry to necro this, but since it was one of the first results when I was looking for info about Maradino's lair and Frederick, I figured it was worth commenting to let people know doing things in the above order bugs the quest.
In my Enhanced Edition game, battling Frederick before talking to Yox the imp resulted in the quest remaining unresolved in the log with no way to complete it. It seems like you need to make sure you talk to Yox BEFORE you go to heal Frederick if unresolved quests make your eye twitch.
I think you get all the XP regardless, though.
