I take it that you are referring to Chapter Sixteen of this excellent novel, when Daniel, Joel and Thacia meet to discuss how they can gain the names of the rich who are going to be away from home at a banquet for Rosh.
Note how this chapter reflects Joel's eagerness to work for Rosh and to do anything that will oppose Roman rule. Thacia's idea is that her brother sells fish to slaves of these rich families, who might reveal what their masters are doing. Thacia then says she will dress up as Joel and go into the city, providing an alibi for Joel whilst he engages in this plan to gain the information Rosh wants. However, Thacia is reluctant to pursue this plan, as she thinks that Jesus would be disappointed in her to discover her involvement in deception:
"It's just that I don't want to face Jesus with a lie. I couldn't bear the way his eyes would look at me."
Thus it is that Thacia and Joel plan to fulfil the request of Josh and remove Joel from being suspected by Thacia herself pretending to be Joel, thus giving him an alibi.