What's the interest rate/monthly cost? Is it currently affordable, given interest rate rises?
Is your relative getting all the benefits they're entitled to? Pension credit is the obvious one if they only have the state pension and even if the award is small, it's a gateway to significant extra financial help.
Providing that they are able/happy to remain in the property, I agree that asking about extending the term might be the least worst option. After all, the lender should be reluctant to see a very elderly person have to move home at their age.
However, if the interest rate has significantly increased, selling and renting/moving into an assisted living facility might be an option.
Have a play with the numbers. If they put the equity into the best savings accounts (including fixed rate products, premium bonds and ISAs to shelter as much as possible from income tax) it should generate a few hundred pounds a month of interest that can be used towards rent, plus they could possibly also safely run down the capital. Plus they don't have to pay the interest. So if they are getting £600 pm in interest and are saving £400 in the cost of the interest only mortgage, that's £1000 pm they can use to rent somewhere, without touching the capital and before they're worse off than their current position.
Moneysaving Expert has some info, but this includes a link to FSA research suggesting 'no significant misselling' which I suppose is fair enough, as no-one can claim that they didn't understand what the term 'interest only' meant and they would have signed at the time for whatever income their advisor told them to state to qualify. So to have a misselling claim upheld, they'd have to admit to lying on their application about their income and/or falsely claim they were of limited capacity.
https://www.moneysavingexpert.com/news/2018/01/interest-only-mortgage-timebomb-act-now-/