I had a think about this.....
It is fine IMO if the sales staff disclosed this. The assistant should have gone on to on selling the food that they are already consuming.
Every time that I have bought fish the assistant always brings up food, and this should be #1 concern for sale of pets/ animals.
There is nothing stopping your mate from going to buy the food that enhances colour and having the fish he saw at the shop.
Just my 2c