To have a £55k shortfall, either the investment vehicle has performed spectacularly badly – or the house is worth at least £200k and they have around £150k of equity in the property (which is more likely).
Given that your parents will not have sufficient time before their retirement to pay off this amount – they still have a number of options:-
Down size – sell the property and buy another with the equity, and be mortgage free.
Down size – as above, but take out a small mortgage to buy something costing a bit more than the equity alone.
Take out an interest only mortgage for the shortfall amount (allowing your parents to continue to live in the property) – when the house is eventually sold, this amount will be paid off from the proceeds.
Sell the property – invest the equity and rent a property.
Seek the advice of a financial adviser – seeking further options.