Can bank information be extracted on a wireless network ?

A business has all Mac computers on an unsecured wireless network. The signal is weak outside the building, but it is detectable. There are about 30 employees. The owner does not want the SSID password protected. Three times in the past twelve months, only one employee has had her checking and credit card accounts compromised with a few very small irregular charges. The bank told her about keyloggers. She hasn't lost money because the bank credited her account, but the bank is also not researching the alleged perpetrators because the amounts are too small. The bank simply changes her account numbers. No other employees have had a problem, and the single employee uses her credit card (linked to her other accounts) in many locations when she's not as work.

It's unknown if she has file sharing turned on.

Can sufficient banking info which can lead to fraud be extracted from a Mac via an unsecured network by monitoring keystrokes wirelessly ?