I would guess that she needs a Visa for the U.S. Brazil is not a part of our Visa Waiver program. So Brazilian citizens do need a visa to visit us. I do not think that it is relevant what the UK has issued to her. As I understand it the Leave to Remain means that she can stay in the UK. That has nothing to do with the United States.
For more information on the U.S. Visa Policy:
http://travel.state.gov/visa/questions/questions_4433.html
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