While Invalidity Pension is taxable as such, tax is not deducted from the payment. If it is their only income, there will be no tax liability on it.
If they had other income for during the relevant time that they failed to declare the pension, then there could be a liability, the amount of liability will depend on how much other income was being earned and what tax band they were in. If the person was already in the higher tax band from the other earnings, then there could have been an underpayment of tax of up to 41% of the value of the pension by failing to declare it.