I've cracked a sternum before while doing CPR. The person lived and was thrilled to be alive with his cracked sternum. It's a sick feeling when you do it to the patient, to be sure, but it does not cause undue suffering in someone who is either able to be resuscitated or who isn't going to make it because those people rarely to never regain consciousness anyway.
If there is a chance of saving someone, you do CPR with the risk of cracking the sternum. Unless they are a DNR.
In this particular case, it sounds like it was a no go from the beginning. From what I've been able to gather, this person may have had obvious chest trauma which made it clear she was dead immediately after impact.