Unsatisfactory service

My youngest daughter has been a dependent like myself on my wife's medical aid for as long as I can remember. She turned 21 last year December and should have been an adults dependent. My wife was billed by her company and for premiums as normal and did not look at the billing. Now Medscheme comes back almost a year later saying that she was billed incorrectly and now must pay almost R25000 in premiums as she was not listed as an adult dependent. Is this a fault of my wife now that Medscheme had not corrected this from the day she turned 21? This is totally unfair and now due to them not doing their job correctly my wife has to pay for it.