Answered by Shaykh Faraz Rabbani
My mother has over 30 years of makeup prayers to perform. Can she give the fidya [expiatory payments] whilst still alive, instead of having to make all those prayers up?
No, this would be invalid by scholarly consensus and there is no textual permission for this from the Qur’an and Sunna.
It is the position of the fuqaha, including all four sunni schools, that all missed prayers and fasts must be made up. The position otherwise is an aberrant (shadhdh) position, held only by Ibn Hazm, Ibn Taymiyya and those (like Shawkani and, in our times, modernists) who tend to follow their aberrations. [For a detailed discussion of the proofs for this, see Reliance of the Traveller
Your mother should:
a) repent from the non-performance,
b) resolve to make up all the missed prayers, and
c) then go about making them up at a pace she can sustain, even if it is a small number daily (such as making up just one prayer with its corresponding current obligatory prayer).
Then, if she dies on this resolve and practice before she manages to make up her prayers, the scholars (such as Shaykh Adib al-Kallas of Damascus) say that it is hoped from the Mercy of Allah that He will overlook the unperformed prayers, because:
a) she had repented from the sin of non-performance;
b) she had resolved to perform them;
c) she was actually taking the means of doing so.
And success is only from Allah.
Walaikum assalam,
Faraz Rabbani.