An account can be put into CNC if hardship is established even if missing return are needed.
Some times just easier to file the return, but not required. They can usually be faxed into ACS.
See INternal Revenue Manual
Open Del Ret modules may be resolved by closing as little or no tax due, or income below filing requirement (P-5-133), if warranted by the facts of the case. See IRM 5.1.11, Delinquent Return Investigations, for options to resolve delinquent return accounts. If the taxpayer is required to file and refuses, a referral or summons may be appropriate. See IRM 25.5, Summons Handbook for summons procedures.
If a hardship determination is verified, a levy cannot be issued or left in place to persuade a taxpayer to file.
Accounts may be reported CNC hardship if a CIS can be verified, even if there are unfiled returns. If the unfiled return is needed to confirm the hardship determination (i.e., income, expenses and/or assets reported on the CIS), the Bal Dues should be held until the return is secured and the CIS can be verified, but levies cannot be issued to collect the Bal Dues if all other income/asset checks appear to confirm hardship.
Ask how they have paid the mortgage, don’t just wonder. Maybe, borrow from family, parents or children.