Didn't file taxes.

In our state there is a property tax rebate program. For some reason my father didn't receive a rebate, but he did receive an application for one. I asked him to gather all his materials and went to submit the form. I just couldn't find his state tax returns. Long story short, his preparer has been submitting his federal returns, but not his state returns.
I do not know why they weren't filing the state returns. He assumed they were. However, the receipts for services only show a charge for federal tax filing, not state. Turns out the last time they filed a state return for him was 2016. At that point, there were no taxes owed and no refund needed.
My father is trying to blame the preparer, but I know this is going to be a dead end since the receipts clearly show they were only paid to do federal returns.
I'm terrified. The majority of his income is SS, but there is an annual RMD. I do not know if federal or state taxes were paid from the actual distribution or if they were paid at the time of filing. He ends up owing about $600-700 each year to the federal government, but even if he owes a fraction of that to the state between fees, penalties, and interest he could end up owing thousands.
What do we do?