Most life cover plans pay out early on terminal illness. He should start the process getting that paid out early. Mortgage protection plans assume an interest rate of 6% per annum, so the cover goes down as if that was the interest rate. As the actual rate has been lower, there should be a surplus amount of life cover that will be paid. This may be enough to cover the tax bill.
The pension should also be payable early at a tax rate of 10%. If left until after death, no tax is paid at all but it has to go through probate, which usually takes about a year. Again, this can be used to discharge the tax liability.
An accountant or someone who deals with the Revenue on a regular basis may be able to advise on dealing with the Revenue. I am sure they will agree a repayment plan if there are no assets to pay the bill outright. Kicking a widow out of her home over a relatively small amount isn't the kind of attention that they want.
I would start with mortgage protection plan and check how much cover there is in place. That could solve the problem.
Steven