Is it permissible for the fiancée to fast the six days (of Shawwal) that were due upon her fiancé who died of an abdominal illness?

If you are one of the deceased's relatives, you may fast on their behalf. However, if you are not, scholars have differed regarding the permissibility of someone other than the guardian fasting. The preponderant opinion is that it is permissible, based on the hadith comparing fasting to a debt that can be repaid by anyone. This is the view preferred by Al-Bukhari. It is better and more cautious to seek permission from the deceased's guardians if you are not a relative, to avoid the disagreement of those who prohibit fasting by non-guardians.

However, if the deceased was unable to fast and died due to an illness that led to their death, and it was an illness from which recovery was hoped, then neither making up the fast nor feeding the poor is obligatory on their behalf. This is unless their illness was incurable, in which case food should be given on their behalf.