There are a lot of people who have never done CPR before who may freak out when they hear ribs being broken. It is not uncommon to break a few ribs while CPR is being performed on the patient. This is especially true when the patient is already old. Its bones may be more brittle than a younger individual.
You should just continue doing CPR with the same consistency and same strength as before. Ribs may heal but it will be hard to bring back life once it is gone. Do take note that performing CPR does not guarantee that a patient will be able to survive.