Thanks for the info.
This time around, they were dealing with an unexpected IRA withdrawal that would have forced them to pay back ~$10,000 in premium tax credits. However, since his income for taking care of my mom turned our to be untaxed, they stayed under the 400% FPL threshold. On the ACA worksheet, the tax preparer included my mom's entire social security income (even the untaxed portion) and the entire IRA withdrawal. She did not include my dad's income. Yet, she tells him to include it in the future when putting in an income estimate for ACA.
The difference will be pretty large. If they don't include it, my mom's $20,000 would be their only income, which would be around 130% FPL. With his income it is closer to 300% FPL.
As far as I know, there aren't any other credits they would be eligible for, just the premium tax credits.