It sounds like they also hired her as casual (i.e. she gets to pick which shifts out of those offered to her she works), with the mutual flexibility they get from that, and then complained about her not working as much as they wanted. If they don't really want a casual employee, and want a part time or full time employee, they shouldn't hire people on a casual contract.
They are clearly not great employers - I definitely won't be giving them my business!