Without more information, there's not much we can tell you. Can the income be stretched over several years? No, not any longer. Could it put them in a higher tax bracket? Maybe, depending on their other income, like pensions and IRA withdrawals. Can the taxes be estimated? Yes, using one of these tools.
https://www.irs.gov/individuals/tax-withholding-estimator
https://turbotax.intuit.com/tax-tools/calculators/taxcaster/
If they owe a high tax bill, will they owe a penalty as well? Possibly. They might want to make estimated payments now, or if they are assessed a penalty later, they can ask for forgiveness (abatement) for cause.
If their SS benefit is their only income it is not taxable. If they have other income, the other income is taxable plus the SS benefit may be partly taxable as well. But it depends on their unique facts and circumstances. If they are concerned, it would be best to talk to a tax pro who can ask all the right questions and get the personal details that matter to their situation.
