I heard that after the application for a work permit to actually get it you have to leave the country and when you get back you present the papers and get the work permit.
For me that is not a problem, but my wife needs a visa to enter the US. So for her spousal work permit, can she do a u turn at the border without having a visa for the US, or can I just take her passport with me?

Or is it necessary that we apply for a US Visa just to get the work permit at the border?

An individual who requires a passport visa to seek admission to Canada (e.g., your wife) is not eligible to process a work permit application at a port of entry. She would need to submit her case to a visa office abroad or inland (if eligible for the latter).