There is no law against someone stopping as long as they are NOT a paid or volunteer provider, the general public is covered under the "good Samaritan law" which releases the general public against any liabilities as long as they are acting in the best interest of the injured person. It was enacted to encourage the public to initiate and provide some care until EMS providers arrive..... Now if they (professional EMS providers) assume care and CPR is continued, then they must have permission from a physician to stop.