Zero people have been successfully sued in either the USA or the UK for doing CPR. In fact the Good Samaritan law protects people from lawsuits in America
You are right not to believe the story about someone suing because of a broken rib. The chances of breaking a persons rib whilst performing CPR is extremely unlikely and again is not cause for a lawsuit.
Sorry to mess the thread with facts instead of urban myths.
British Heart Foundation - Hands-only CPR FAQs
http://www.life1st.com/files/CPR-Legal_and_Ethical.pdf