Is it obligatory for a driver who accidentally killed a person, and whose victim's guardians pardoned the blood money (diyah), to fast for two consecutive months, or is he exempted due to his weakness and not having intended harm?

If it is established that the killing was accidental, then the blood money (diyya) and expiation (kaffarah) are obligatory, even if the driver did not intend to kill. If the guardian of the victim waives the blood money, then the blood money is dropped, but the expiation remains, which is fasting for two consecutive months. If one is currently unable to fast consecutively but believes they will be able to in the future, they should delay the fasting. If they despair of being able to fast consecutively, they should fast for two months to the best of their ability, due to the generality of Allah's saying: (Allah does not charge a soul except [with that within] its capacity) and (He has not placed upon you in religion any difficulty) and (So fear Allah as much as you are able), and the saying of the Prophet, peace and blessings be upon him: "If I command you to do something, then do of it what you are able."