Qadhas and backache

Respected Mufti Saap, an elderly female in her early 60s has decades of qaza umri to read and is experiencing difficulties in standing and reading after about 15 minutes when her back starts to ache. Bear in mind that her daily fardh salaat is read as normal ie stand and read.She used to sit on a chair and continue reading after tiredness and backache but was advised that her reason to sit and read was not valid and therefore risked her salaat not being accepted. Kindly advise