the parabolic antenna needs to be aligned very accurately to get a decent gain. It is very unlikely she can do that with such a simple support structure at 3km distance.

even if she can do that, she needs to remove the original antenna of the router and connect a high quality coaxial cable from the router to a small feeder placed accurately at the focus of the parabolic antenna.

the connection and the feeder needs to be designed

here is a DIY parabolic wi-fi antenna which is more likely to work: http://binarywolf.com/249/parabolic.htm