This pretty much, the scenario is way to vague to offer any definitive answer. Generally speaking, the person will be handicapped by their bottom ranking, but they can boost their ATAR a fair bit with a strong external performance. Realistically, it's unlikely that a person coming last is going to suddenly wake up after bludging and/or doing poorly for an entire year and turn things around on the HSC. Turnarounds are possible from a technical standpoint, but from a realistic perspective, the chances are fairly remote.