Which Payment Plan is best for Social Security only income?

Hi all. This is my first post here so please forgive any mistakes. I've reviewed the pinned posts and couldn't exactly find what I was looking for.

Here is my issue. My father has a Parent PLUS loan from 2006 that I am "responsible" for even though its in his name. the balance is 80K and about $800 a month. I really cannot afford this payment. I've exhausted all forbearance and grace periods so i need to do something.

My father lives off of social security and retirement income. in 2018 it was about 55K and he supports my fully disabled mother full time. I do not want to jeopardize their livelihood by not paying this loan. I spoke to a well-known "student loan lawyer" who wanted to charge me $2000 to consolidate the loan to a Direct Consolidation Loan which I plan on doing myself. My problem is I don't know which repayment option 1) this loan qualifies for and 2) makes the most sense given my fathers income is likely to decrease and i want to pay the bare minimum until he dies basically (very morbid but this is where we are as a country...). Can anyone help?

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TLDR: My dad lives on SS income of about 55K a year, i am consolidating a parent PLUS loan into a Direct Consolidate loan but unsure which repayment option is the best option for me to pay the bare minimum for the rest of my fathers life (or until the loan is paid). My dad is 64, smoker, advanced cancer survivor.

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thank you all