Personally I have never found an EKG to be helpful-no findings just means nothing noted at the moment of testing-but it is an easy money-maker...
Does she have scoliosis/kyphosis? Has she been followed by an orthopaedist/neurosurgeon for her spine? Seeing a specialist for followup of her spine, instead of primary care, could kickstart an investigation into her current symptoms since spinal curvature affects the chest.
I have random zaps of chest pain/left arm pain, along with high blood pressure, cholesterol, family history, etc. My two EKG's, one stress, were normal. The random zaps occur during my couch potato moments, but traveling/walking into a hospital for testing with all the attendant stress/exercise is anything but a couch potato moment, so it doesn't surprise me when they record nothing unusual.
May come from neck problem.
Many people can't get over the idea that their symptoms are caused by their fear. I can't diagnose her from here, but that is a possibility since the EKG says no problem.
Over the years, I get chest pains when I worry, but I have had a few routine EKGs which say I am fine, so whenever I get a chest pain I take a deep breath and tell myself nothing is wrong and the pain goes away - so it mat be some kind of mental issue for her perhaps coupled with sore actual chest muscles soreness that makes it seem like dangerous pain. If she is in fear that something dire will happen then she can be tensing up and actually causing the symptom.
Continue to work with you docs, however I would do some reading about how to calm down when worried about her heart to see if that helps or gets rid of it.