refusal to exchange goods

my father in law purchased the incorrect milk for my child 19/01/2016,today i went back to exchange the sealed tin of milk for the correct one and was turned away by manager Rasta who stated it is the law , my dad in law can't read and the cashier didnt inform him of the return policy, why does spar not cater for all customers because my father has now lost R234, which could of been avoided if Spar catered for all customers, both tins look similar and Spar didnt have the correct nido therefore my father in law didnt realise that there was two different nido number 3. i am disgusted by the way Spar treats customers, robs people like this.