You can't. It sounds like when they applied their estimated income was likely below 200%.  Between 100-200% the credit would be higher,  so at 242%, they would not be eligible for the full credit.  Which means, if they received the full credit amount, they would have to pay it back.  However, the repayment amount should be capped at $975 if they are single and $1,950 for any other status. 

I knew what you're both saying here.   I just didn't understand why a low-income taxpayer's not being a dependent triggered the repayment requirement.  I've read the IRS information again and dependents aren't the ones who repay, only independent taxpayers.  Clicking the "non-dependent" choice created the opportunity for liability.   The explanation on line 29 of form 8962 in ProSeries says "If income exceeds 400% of [federal poverty level}, then some of the {advanced premium tax credit} must be repaid."   That apparently does NOT mean that people below the 400% level are exempt from repayment.   If they'd declared their income was going to be lower than it ended up being, no matter how low their income was, they're required to repay.   

This "insurance" has been a useless, sneaky trap since 2012.  I was familiar with repayment, just not these two quirks about it.  Both of you were right, you just weren't answering what I was asking.