Your parents might miss out on certain tax credits if they don’t file taxes married jointly.
They should try and get a different tax preparer for 2019 taxes and see if they can amend the last few years’ returns, if they were married.
If they are low income, see if you can find a VITA center, there are volunteers there helping you with tax preparation.
https://www.irs.gov/individuals/find-a-location-for-free-tax-prep