How can a woman and her husband atone for the sin they committed after the husband broke his oath, sworn upon the Holy Quran, to quit smoking?

The husband must repent from smoking. He is also required to pay expiation for his oath, as stated in the verse: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But he who finds not [the means to do so] - then a fast of three days. That is the expiation for oaths when you have sworn. And guard your oaths. Thus does Allah make clear to you His verses that you may be grateful." (Al-Ma'idah: 89).