My mother in law goes through this with my sister in law, who thankfully does not have children, but is 54 and has never been gainfully employed. mother in law is 83.
I finally got mother in law to at least stop giving her cash for the bills. Now sister in law is required to give mother in law the actual bill that is overdue or the notice or whatever and mother in law pays it directly. She also does not give her anything extra that month if she has to front her a bill.
It took me 10 years to get mother in law to this point. One day, I just sat her down and compared her Medicare and crappy other insurance to the plan I had at work and the top notch plan she was paying for for her D (who was MARRIED at the time! but didn't like her then H's insurance!). She finally downgraded her insurance and as of this time, doesn't pay for it at all anymore - though now, you and I do because this girl now gets public assistance! But at least my mother in law doesn't have to worry about eating dog food for dinner.
Maybe you should just pay the bill directly if you wish to. I feel badly for the little girl about the medications, but I wonder why it was too close to the insurance time again - is the medication something difficult child daughter could resell for a profit?