Document, document, document!!!
In writing. Names, dates, situations.
You will spend most of your time doing this - but it will become necessary in the long run.
Due to the theft, suggest in writing to the boss that a camera (locked) be installed in the cash room. That is just good policy.
Get an audit trail established. Follow it.
If you are her supervisor, and your boss is not supporting you - you have an issue with your boss. A conversation may be in order. Document that, as well.
I wish you well. ((hugs))