Earlier quoted context omitted.
Your friend is lying to you. If you do this enough times, the bank automatically flags the account for money laundering. It's very basic and standard money laundering procedures. "I don't believe it'd be realistically possible to stop this from occurring." - It already happens and is already stopped so you're wrong on that one.
How is it money laundering when the money is already in the bank and thus accounted for?
Mob boss starts a building company, paying the employees with dirty cash (but hey, no income tax, so no ones complains).
Building company charges customers, but asks for it in cash.
Dirty money goes out to construction employees, clean cash comes in from customers. The Bank flag is just saying there is laundering occurring near the money.