Is it the customers or shop keeper's responsibility to check if a product is safe?

My cousin brought a milkshake from a shop and once she took a sip, she threw up; it was 8 days out of date.

She returned to the shop to inquire about the said product only for the Shop Keeper to bluntly say “That is not our problem because the onus is on the customer to check if a product is passed its sell by date before buying”