What I ended up going with was increasing skittishness in the rat familiar. When the magician first arrived at The Silvery Eel, the rat seemed a little agitated. Its anxiety became more pronounced as the party delved deeper. When they finally confronted the gnagana, the familiar failed a morale check and fled, depriving the magician of its bonus hit points. It did eventually return after the gnagana had been destroyed, because I didn’t want to screw a 1st-level PC too badly.