Wait - is she buying YOUR chicks to feed to her snake? Or, just feeder chicks in general?
If it's your chicks that she's buying, then I think you have a right to be mad if you believe she's aware that you sell your chicks to be pets/egg-layers.
However, if she just told you in passing that her pet snake eats chicks, I don't see the problem. That is a plain and simple truth. Many pet snakes eat chicks. It is the way of the world. Some snake breeds prefer birds to rodents and are picky enough to starve themselves if not fed birds. Many people and places breed chicks and ducklings specifically to be fed to snakes. That's nature. People eat chicken too. People who eat "cornish game hens" are just eating baby chickens. I see no difference.