I’m puzzled by the idea of “a milkshake” being out of date. Where I come from, milkshakes are made at the time of purchase, from fresh ingredients. I take it that this was some kind of packaged milkshake, which I’ve never seen before.
“In general” responsible store owners and managers check the expiration dates of perishable products and remove them from sales displays when the date has passed. But it’s certainly conceivable that in a large store, for example, or with new or overworked staff (or product mistakenly replaced in a wrong area, for example) that some goods can be sold that shouldn’t be.
The ultimate responsibility certainly rests with the consumer – always.
For one thing, with the example that you cite, unless your cousin returns the the store very soon after the incident, with the product in hand – and the receipt that shows that it was purchased recently – then how will the store owner know that it’s his fault, and not someone just trying to replace a product that sat on his own refrigerator shelf at home for a week or longer?
I’ve never seen or met a store manager who, presented with that evidence of a bad act on his part, wouldn’t take every reasonable measure to satisfy the customer and change his processes to ensure that it doesn’t happen again.