What sort of guarantee do you offer for a leather repair or re-collect a customer who wants a written 12 month guarantee off me for a re-colour

Offered her 3 months but she is insisting on 12 month think I will just walk as her suite is only 14 months old and in a bit of a state.She says she has only used the leather kit supplied with suite which I find hard to believe as the amount of colour that has been removed beggars belief