If she had a miscarriage there would have no longer been a fetus so no heartbeat. An incomplete miscarriage would have left behind enough tissue to cause septicemia. A D&C might have been in order but even in an incomplete miscarriage there is no heartbeat or viable fetus. Someone has really confused this story.
But there may be a heartbeat in an "inevitable miscarriage" (bleeding & dilation, but the fetus remains in the uterus). And sometimes those do not progress. That can cause significant risk of infection leading to septicemia. It's possible that's what happened here.