I am afraid, I won't be able to give any definitive answer because till one sees the patient and goes through the complete clinical records, it is impossible to give a firm opinion.
In general however, pain the back following any open heart surgery is extremely common because when we open the middle of the breast bone and retract the bones to reach the heart, we also crack and displace certain ribs on the back side near the spinal bone and some of these fractures do not even unite. Therefore the pain between the shoulder blade and on the sides could be related to that also.
However, some times the pain could be related to the original pathology also e.g. if the original operation was done for enlargement of the aorta, then the aorta which has been left behind, can also enlarge or even dissect and can give rise to pain in the back. However, these things should have been picked by an MRI scan, which you have written has come normal. I therefore would tend to believe, it is the former problem of pain coming from bones and joints and not from the heart or blood vessels. An important third cause for this kind of a pain could be totally unrelated problem with either the pancreas or the spinal column itself and therefore if the cardiologist has ruled out any heart related problem then you must consult a general surgeon/orthopaedic surgeon for their evaluation.