As I have said ,You said no worries you will simply replace the tyres , so thats what he thought was happening , you never asked him to keep them,
Plus why should i advise him all i knew was you were going to replace them ,,why should i have doupted this and started to worry about proof .

As i keep saying i am only getting involved because i know the full story ,
But if you dont want to sort it then no loss to me ,, you brought it on here not me .
As for the other comment from davS2 , I do see the point but if i say to a customer dont worry i will replace it i do , and i would hope my word is enough for my customers, and i am shure it is ,if i wanted proof of faulty goods i would advise the customer as soon as i know there is a problem ,