She must have a multi-entry US

visa, as indicated by a letter M on her visa. If it's a single entry visa or or multi entry visa that has expired (this is not the same as the visa expiration date. This is the last date she can reenter US), she will not be able to return to US. If she has a US multi-entry visa, she will have to apply for a Canadian visa (even for one day) and there is an application fee (it used to be $75 US dollars).