She has to offer expiation for killing but she cannot free a slave or fast, so can she feed a number of poor persons?
Question

                There is a woman who killed another woman by accident and when she asked about the ruling, they told her that she has to feed sixty poor persons, but it seems that this fatwa is not correct, and she has to fast for two months. But now she is elderly and cannot fast, so what should she do?.
Praise be to Allah.The expiation for killing 
    someone by accident is to free a slave. If that is not possible then it is 
    fasting for two consecutive months. The one who cannot do that does not have 
    to do anything, and feeding the poor is not an option in this case, 
    according to the more sound opinion. 
Allaah, may He be exalted, 
    says (interpretation of the meaning): 
“It is not for a 
    believer to kill a believer except (that it be) by mistake; and whosoever 
    kills a believer by mistake, (it is ordained that) he must set free a 
    believing slave and a compensation (blood money, i.e. Diya) be given to the 
    deceased’s family unless they remit it. ... And whoso finds this (the 
    penance of freeing a slave) beyond his means, he must fast for two 
    consecutive months in order to seek repentance from Allaah. And Allaah is 
    Ever All‑Knowing, All‑Wise”
[al-Nisa’ 4:92]
The scholars of the 
    Standing Committee for Issuing Fatwas were asked: Do I have to fast after 
    paying the diyah and how many days should I fast? Are the fasts to be 
    observed consecutively or can they be broken up, or can I give food 
    instead? 
They replied: You have to 
    offer expiation for killing by accident, which is freeing a believing slave. 
    If you cannot do that, then fast for two consecutive months. It is not valid 
    to break up the fasts, and feeding the poor instead is not acceptable in the 
    case of expiation for accidental killing, because there is no evidence for 
    that with regard to expiation for killing in the Book of Allaah or in the 
    Sunnah of His Messenger  (peace and blessings of Allaah be upon him), 
    and your Lord is never forgetful. End quote. Fataawa al-Lajnah 
    al-Daa’imah (21/273). 
Shaykh Ibn ‘Uthaymeen (may 
    Allaah have mercy on him) was asked: There is a man who has diabetes, and he 
    has to offer expiation by fasting for two months – the expiation for 
    accidental killing – but he cannot fast because of his sickness. What should 
    he do? 
He replied: If he is unable 
    to fast then he does not have to do anything, because the expiation for 
    accidental killing can only be freeing a slave or fasting for two 
    consecutive months, as it says in Soorat al-Nisa’ (interpretation of the 
    meaning): “And whoso finds this (the penance of freeing a slave) beyond 
    his means, he must fast for two consecutive months in order to seek 
    repentance from Allaah. And Allaah is Ever All‑Knowing, All‑Wise” [al-Nisa’ 
    4:92]. Feeding the poor is not an option here. If he is able to fast 
    them he must fast, otherwise it is waived in his case. 
End quote from Liqa’ 
    al-Baab il-Maftooh 107, question no. 24. 
He was also asked: 
There is a person who has 
    to offer expiation for killing, and he cannot free a slave or fast. Can he 
    feed the poor instead, or is expiation waived in his case? 
He replied: No, it is 
    waived in his case. The expiation for killing is of two types only: either 
    freeing a slave or fasting, because Allaah, may He be exalted, did not 
    mention a third option. If the third option had been obligatory then Allaah 
    would have mentioned it, as He mentioned it in the case of expiation for 
    zihaar (a jaahili form of divorce): freeing a slave, and if that is not 
    possible then fasting for two consecutive months, and if that is not 
    possible then feeding sixty poor persons. But feeding the poor is not 
    mentioned with regard to expiation for killing. Based on that, we say that 
    if a person is unable to free a slave or fast, then it is waived in his 
    case. End quote from Liqaa’ al-Baab al-Maftooh 190, question no, 15. 
And Allaah knows best.