I agree with Harris, check with the physician, but it could be shin splints. They can sometimes appear like bruising. If she is cleared by her md (to make sure it is nothing more serious such as a blood disorder, compartment syndrome or stress fracture, to name just a few) and it is shin splints she should dial down the HIIT, add ice massage (to the shins after workouts), calf stretches and check out her shoes. You should also check her alignment; does she pronate/supinate etc. You may need to address some imbalances.