If I marry a girl who is in the US on an H-2A worker visa, can she legally stay in the US?

I want to marry a beautiful girl I have been dating for several months. She is in the US on a H-2A (agricultural worker) visa. If we marry before this visa expires, will she be able to legally stay in the US? Or will she first have to return to her home country and reapply? Any other information that might be helpful is greately appreciated.