It is in Chapter 20 that Joel tells Daniel, his friend, that his father has had a change of heart concerning his son's friendship with Daniel, whom he knows to be a rebel working in league with Rosh. Of course, what has triggered this change is the fact that Daniel saved Joel's life when he is captured, in his disguise, by the Romans. Note what Joel tells Daniel when Daniel suspects Joel's father despises him:
For saving me? He sent you a message. He says you are not to hide in the passageway again. You're to come to the house whenever you like.
Joel's father's change of heart therefore is something that is triggered by the way that Daniel did everything he could to rescue Joel, even risking his own life. No matter how zealous Joel's father was in his beliefs, when his own son's life was at risk, he recognises that love overpowers any ideology. This is something that Daniel himself learns by the end of this story, and therefore Joel's father's change of heart is something that could be said to foreshadow Daniel's own change of heart towards the Romans, expressed in his invitation to Marcus to enter his home as a guest.