Dad passed away with nothing and now the irs is asking for $30k

My father passed away a few years back sick and broke. He lost his house and any real assets along with his health do to a hit n run. He died with a car and a few hundread dollars in his acct and simple household posessions and medical debt. His only real asset is a couple acres of undevelopable land that he and his sister inheirited. They were both on the title and she is still living.

My problem:
3 years later the irs sent me a letter claiming he owes $30k for years he quit filing returns. The amount owed is a guestimate based off of his previous returns prior to leaving work. The years in question by the IRS are years that he was out of work, and recieving disability. What do I do, how to i get the irs to acknowlede he was out of work and not skippong oit on taxes? ive sent copies of his disability etc. And havent heard a single word back in 4 months. They continue to send letters asking for the balance, so what ive been able to do is getting me nowhere, does anyone have any insight ? Any help is greatly apreciated as I am disabled with a slew of condtions, i do t have the money or energey to fight this.

Thanks for any info or help.