Why is Rinaldo in the chapel when Fabrizio arrive to create the illusion that will prove the count's guilt in the novel, Midnight Magic?

When Fabrizio arrives at the chapel to create the illusion, he finds Rinaldo, who is really the prince in disguise. Rinaldo has destroyed the candles in order to appear himself as the ghost and force a confession from the count. The prince, however, has injured his leg, leading Fabrizio to stand in his place and force the confession that leads to the counts arrest.