please help with strange mother B2 denial

My mom came in 2003 and got 1 year stay (most people are given half a year) when she entered the US, she stayed a year and went back to China. Her B2 visa was denied yesterday by the Guangzhou Consulate. They said she stayed in the US illegally for at least half a year, because they didn't believe 1 year of stay can ever be possible. My mom was so upset, because even if it was a mistake, it is a mistake made by the US officials, how could she know she can only stay half a year when 1 year's stay was given.
We don't know if one year stay for B2 was ever given lawfully or it was merely a mistake made by the INS. And how can we get around this?
Any suggestions will be greatly appreciated!