If someone dies owing missed fasts, what should be done?

Narrated from Ummul Momineen Aisha Siddiqa (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever dies and has obligatory fasts to make up, his heir should fast on his behalf."

Narrated Ibn Abbas (may Allah be pleased with them) that a man came to the Prophet (peace and blessings of Allah be upon him) and said, "O Messenger of Allah! My mother has passed away and she had a month's fasts due. Should I fast on her behalf to make up for them?" The Prophet (peace be upon him) replied: "Yes, fulfilling Allah's debt is most deserving."