Not yet sure what all is available here. When I first encountered Frederick I was near the docks in Amsterdam and he was tied up in some sort of Freedom Movement for the liberation of the Netherlands. You can choose to help him with some of his logistical problems and you wind up back in Amsterdam with an associate of Frederick's named Hoorn, Frederick is nowhere to be found. (I will post this particular encounter and dialogue in the Quests, Adventures and Battles section of this Wiki.)