Roxanne promised Luke that she would finish their report while he was on vacation; however, the deadline for that report was postponed  Clearly, if you promised a friend that you would meet them for lunch but just before lunch you felt ill, it would not be wrong for you to miss the lunch; your friend would not expect you to be there if you felt ill
Similarly,
if Luke would not expect Roxanne to finish the report under the circumstances, then it would not be wrong for Roxanne to fail to finish it