No problem
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Just assure her it's definitely Sheffield and that she must NOT send anything to NYC - it will just be a waste of money and will end up delaying her application even more.
Last February, we had someone here on the forum who applied online end of January and mailed her application early Feb. At the time, spousal visas were being processed within a week and so if she had mailed it to Sheffield, she would have got the visa before the end of February.
However, the online application/biometrics confirmation (incorrectly) told her to mail the documents to NYC, which she did. It took them a month to return them back to her, by which time, the processing times in Sheffield had increased to more than a month. By the time her application arrived there, the processing times were at 2 months and counting. So, the visa she should have received in February 2014 was finally issued in May 2014... that one small mistake caused a processing delay of 3 months!