Part of filing for a suit is proving that you have the right to represent the individual. Usually this is easy, as most people are recognisable (leaglly) as representing themselves.

As the individual is officially deceased, you need some documentation from before they were recognised as such, granting their rights to a recognisable individual.

That is, the lawyer needs to be granted the right to represent her, but she probably doesn't currently have the necessary legal paperwork to give them that. She can't represent her own estate - she's legally dead.