Upvote:2
I think it would be less likely she would be granted a visa if she applied for it in the Netherlands instead of her home country.
Unlike many countries, the US will allow people to apply for a nonimmigrant visa from any country they're legally present in, even as a tourist (unless they've had a previous visa cancelled at the border, then they must do it from their home country). So, she can apply for it there.
The problem is twofold: First, that the US embassy there will find it much more difficult to verify any relevant documents from Nicaragua, and if they are unable to verify information, the visa could be refused. Second, that she might not have the documents with her to begin with!
Then again, if she is relying on her boyfriend to support her, it may be better to apply for the visa in the Netherlands, as his information would be more relevant, and easier to verify from the Netherlands. But this would be a stronger case if they were already married.