life insurance beneficiary rules

Is there a law as to when they are not allowed to rename in life insurance when a terminal illness?
I am in court case.My husband died a rare tumor.He was heavy morphine and other painkillers for severe pain in the law says hospital.Where draw the line at the evolution of a beneficiary? Especially in this situation …! Additionally, no officer was to see his condition.But someone who works with the insurance company was told (by an employee in the company of my husband working on) that can not be capable of writing and the (insurance company) the person signed on behalf of my husband. My opinion is Japan.But must be a universal rule … Is there a law? Or the number of law as a reference? Help please ….
consult your attorney mam thats serious