Going through a debt relief program is not a requirement for filing a tax return.
You can only file taxes for the last three years, so your friend could file 2023, 2022, 2021 and 2020.
It may end up that your friend is due a refund for all those years, which may be why they never heard from the IRS.
If they owed tax, the IRS has an Installment Plan program where you can pay a minimal amount monthly.
A debt relief program helps in consolidating all your debts into a manageable payment amount, which may be helpful in the event they ended up owing a large amount of tax.
I would suggest they prepare the returns and see where they're at before filing. In this case, they may want to use the services of a professional tax preparer.
Here's more info on how to Find a Verified Tax Pro.