lung cancerl

My sister had breast cancer 3 years ago.  In the past 2 weeks she started to cough a lot and has shortness of breath.  She went to cardiologist who had an xray done.  It showed nodules in her lungs.  Then she was sent to an oncologist who had a CT Scan and a Pet Scan.  The Dr. says the Pet Scan shows her cancer has metastized in her lungs.  Is that PET Scan the most accurate to determine if it is cancer or should she get a second opinion.If she should get a second opinion would it be on what kind of treatment or the reading of the PET Scan.  Also her breast cancer was a triple negative.  How will this affect her treatment and recovery?