I have let it go, though I never received the vouchers... Its not my fault people keep replying!
I had already made the complaint before mentioning it to my colleague. As a manager myself, I cannot tolerate this kind of behaviour towards staff. I have to say that even if I hadn't heard the conversation this is a very bad way to be treating staff and if I was paraded and shouted at in public by my boss I too might very well feel like resigning rather than continuing to work in such an environment