What do you Have to Pay Back If you Make too Much for your Cost Assistance?

I work as a nurse for a private insurance company, but I was asked a question about Obamacare that I could not answer. If someone started out with Obamacare in 2014 because they qualified and then, later in the year, no longer qualified and dropped Obamacare due to a job change, will she be required to pay back claims she had from hospitalization in 2014?  This lady has absolutely no idea how to manage her healthcare and dropped here ObamaCare coverage this month.