In the US:
When you pass away, your mortgage doesn’t suddenly disappear. Your mortgage lender still needs to be repaid and could foreclose on your home if that doesn’t happen. In most cases, the responsibility of the mortgage will be passed to the beneficiary of the home if there is a will.
https://www.bankrate.com/mortgages/what-happens-to-mortgage-when-you-die/
This partially depends on how much the house is worth now. If it's somehow a million dollar house today, and the amount owed is $250K, then you could refinance that and make payments. This would be a reasonable transaction.
But if you have negative equity - if you owe more than the house is worth - the best thing to do right now is stop paying that mortgage. In the short term, you're just throwing money away, because that mortgage is never going to be paid off. In the longer term, see the above. Stop paying the mortgage.
Yes, this will end in foreclosure. Yes, you'll "lose the house," but honestly, it was lost a long time ago, you just didn't know it. Once the foreclosure has happened, your grandmother will be relieved of that debt, because the home was collateral. Foreclosure is highly likely to happen anyway after your grandmother passes, but then it becomes infintely more complicated, especially if there are multiple beneficiaries of the property.
My in-laws were in a similar situation long ago. They had gotten a now-illegal "reverse amoritization mortgage," which basically guaranteed that their mortgage payments would always be less than the interest accrued. Same kind of result: they owed way more on the house than it was worth, way more than they had originally borrowed, and were essentially never going to get out from under it. The advice they took was exactly as above. Stop paying the mortgage.