Please help. A consumer is getting stonewalled about a $751 charge.

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It is highly unusual for a corporation to stonewall the Haggler. But that began to happen two months ago, when the Haggler contacted a Texas-based electronics and appliance chain called Conn’s, which has more than 80 stores, mostly in the South and Southwest. He wrote on behalf of a customer named Grace Bunmi Salako Smith. She is a Nigerian immigrant with a Ph.D. in public health who lives in Dallas, where she teaches children in juvenile detention.

Last year, Ms. Smith went to a Conn’s in Dallas and bought a refrigerator and a computer, for a total of $2,587. She says that she could have bought this merchandise outright, but that she wanted to enhance her credit score so she would have better standing to make investments in real estate. So she financed it with a special offer from Conn’s, the terms of which allow her to pay no interest as long as she makes regular payments each month.

Ms. Smith says she abided by those terms, though the story becomes complicated. It’s clear that Ms. Smith made at least one late payment, as a protest because no one at Conn’s would service her then-broken refrigerator. But that late payment was explicitly forgiven by the company, as detailed in a Conn’s document that Ms. Smith scanned and forwarded to the Haggler.

In April, when she went to Conn’s to make her final payment, and close out her balance, Ms. Smith was informed that she owed $751 in interest. According to Conn’s, she had broken the terms of that zero-interest program.

So here is the question: Is Conn’s correct?
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More: http://www.nytimes.com/2014/07/06/your-money/the-haggler-how-to-pierce-a-deafening-silence-maybe-an-army-can-help.html

Please contact them in some way.
Mrs.Smith deserves to get a clear answer from them especially since she has a document forgiving the late payment. I don't know how or if that affects her original contract.

They may be right, but ignoring her is wrong and disrespectful.