There was quite a lot on the different kinds and levels of dishonesty in a book I read (listened to) recently, called
Predictably Irrational.
One finding, consistent across several studies, was that people were more likely to cheat or steal the farther they were removed from actual cash money. They would steal a piece of candy worth 25 cents, but not a quarter from the same location and situation. When given an opportunity to lie about how much money they were supposed to receive for a task, people lied to a certain degree to the person handing out cash, but lied far more if they were being given tokens that were then traded for cash at a different location, even if the exchange took place immediately, just a few feet down the table, in view of the person who had done the token exchange.
People also stole more, and more often, when given an opportunity to do so electronically rather than in hard cash, even when the odds of getting caught were equal. Changing a number on a screen did not feel as bad as taking physical cash, yet somehow taking a physical
thing from someone also didn't feel nearly as bad as taking physical cash.
Another fascinating finding was that people could be "primed" to behave more honestly with a seemingly unconnected reminder about morality beforehand. Various groups were given a writing task, and then the opportunity to cheat on a test. When the group was told to write down all the books they were assigned to read in high school, they cheated a certain amount, but when the group was told to write down as many of the Ten Commandments as they could remember, they cheated
far less, even those who were avowed atheists and/or could only remember 1 or 2 of the commandments.
According to the author of the book, your colleague should have put a small sign in front of the bowl with a solid quote about honor or honesty, preferably attributed to a well-known person (even if they never said it,) and she would not have been stolen from nearly as much.