The questions are why was she staying there and what approvals, if any, did she receive?
If she just decided that she could stay there because it was empty, then she should be fired. That’s probably the case. However…
If she got approval from higher-ups… then the issue is why they granted her approval. Could an argument be made that there was some value to the bank in having her living there? It’s conceivable. But it’s a stretch.