Your friend has to know if his/her current visa is for single entry or for multiple entries. If it is for single entry, your friend will not be able to come back to Canada even if his/her visa is not expired, unless he or she gets a new visa. It will be very difficult for your friend to get another vistor's visa in China, because his/her application of the green card is in process. If his/her green card is approved in future, he/she will be able come to Canada as an immigrant, of course.
BTW, if your friend really has urgent things to handle in China and can trust me, I can help. I am 闲得慌.