How much of the Social Security will be taxable?  Are they collecting $40,000 a year and paying tax on $34,000 of it?  Or $18,000 a year and paying tax on $9,000 of it, at 10%?  

I just had to lecture a 66-year-old client about not waiting to start.  He has been retired from a public-safety job for five years; his Social Security is not going to be much, but more than it would have been before the January change repealing pension offsets.  Take $1,000 a month now, or $1,100 a month later?  You don't break even until you're 85.  Were you doing the returns for these clients in 2022?  Did you ask them each year about their Social Security plans?

I would file all the returns, then arrange a payment plan to cover all years.  Also, do they have no credit?  Better to owe the bank, than the government.