HMO as primary insurance and Medicare as secondary?

My friends Mum has an HMO as her primary insurance as she is still working and Medicare as her secondary insurance. She has had the HMO for 5+years and they have been automatically deducting her HMO monthly payment until January 2010. Now they are saying that she must pay them $600 as she hasn't paid her monthly payments. They will not give her a reason as to why they stopped auto-deducting when she did not change banks or stop the auto deduct herself. She has tried contacting the HMO to no avail. Please help me guide her on what her next step should be. She keeps getting the run around from corporate office and only has a number she can call no local office. Is she responsible for this $600 as this was not her fault for the mistake - it was the company who made the mistake? Thank you for your help.