Is it permissible for my mother, who suffers from rheumatoid arthritis and whose paid sick leave has expired, to take another leave for a different illness approved by the Ministry, given her severe and progressively worsening pain from the disease, and considering that she would lose her rights if she settled her pension or took unpaid leave?

It is not permissible for an employee to request a false sick leave due to the involvement of lying, forgery, and unjustly consuming wealth, as well as inviting the doctor to bear false witness. These are grave prohibitions, based on the Almighty's saying: ﴿So shun lying words﴾ [Al-Hajj: 30], and the Prophet, peace and blessings be upon him, said: "Beware of lying..." and he also said: "Indeed, false speech and false witness." Both the employee and the doctor who grants the false leave incur sin. Your mother's health conditions do not justify these prohibitions. Instead, she should inform her workplace of her true situation and request leave, or discuss with her manager reducing her workload, or transferring to another department, or taking unpaid leave, or settling her pension.