DFS being difficult

My sister bought a sofa from DFS ten months ago under one of their buy now, pay later type schemes. The sofa is cream and black and within a few weeks of it being delivered, the black colour was coming off onto the cream. DFS were contacted and they repaired it. It has happened again since on two more occasions and on all three occasions, my sister has had to take time off work to accommodate them. She now has no more time available to take off from work thanks to this but DFS are refusing to come on a Saturday to remedy the situation, instead insisting she must make herself available during the week. My sister is a nanny and therefore works very long hours so an evening in the week is out of the question. DFS will not refund nor discount the sofa and refuse to budge from their weekday only offer.

What, if anything, can be done? Ideally, my sister would like to get out of the contract altogether and go elsewhere but she's aware this is probably impossible. Does she have any rights?

thanks