My dependent parents lived in my house and I paid for all their expenses. My dad has few hundred dollar interest income. I met all the requirement for using them as my dependents except one- When my dad filed the tax, he mistakenly filed joint with my mom. But he mentioned that someone can claim them as dependents. This was an honest mistake. Unfortunately, they can't amend it backward from joint to separate after April 16. But I really need to use them to qualify for HOH and that makes a big difference. Any idea?

I'm curious why your father even bothered to file a tax return if all he had was a few hundred dollars in interest income. He had no pension income, no social security, no other wages? Remember that you can't claim any dependent who had over $3650 in income.

Sorry - I have no suggestions as to how to rectify the situation.

If your father file the joint return only to get withheld taxes back and they don't have a tax liability, you can still claim your parents as dependents

Joint return test.
You can’t claim a married person as dependent if he or she files a joint return except that the joint return is only a claim for refund and no tax liability would exist for either spouse on separate returns. Your U.S. Tax Return: Requirements for claiming a dependent

Your father must have claimed personal exemption on the joint return. Thus if your mother's income is more than $9350, you can not claim your father as dependent.