Is it possible for a Muslim to change for the better? Will Allah reward me for having shielded a Muslim woman, knowing that my wife and I are striving to increase in piety and faith?

The state of a Muslim may change from shortcomings to repentance and righteousness, and their condition may transform from wrongdoing to goodness and closeness to Allah. The same can happen to a disbeliever, as the Prophet, peace and blessings be upon him, said: "Indeed, a man may do the deeds of the people of Hell until there is nothing between him and it but a cubit, then the decree overtakes him, and he does the deeds of the people of Paradise, and enters it." So, praise Allah for granting you a righteous wife and conceal what she told you, for repentance erases what came before it.