Useless accounting department

I received an account for my mom, 90 years old, in October 2019. I paid the account immediately, but since then she gets harassed for that amount, although I have send proof of payment several times, and have called them, and was told that amount was wrongly allocated and would be rectify, but nothing happened. They have given the account over to vericredit, and I sent the same POP, and this lady told me she could not provide details and referred me back to Ampath. I asked for a statement where that amount shows on my moms account - nothing. Now what am I suppose to do now.