Application on Compassionate/humanitarian grounds

I am presently a community support worker having migrated to Canada in 2006. Well this forum had been of immense help to me providing timely and very sensible advice then.

Now, this question is for the parents of a client of mine who has chronic schizophrenia and living alone here on disability benefits. I understand his parents who are in U.S.A and green card holders could apply for PR on humanitarian grounds to take care of the son. Correct me if I am wrong on this.

If I am correct and the parents choose to apply for PR on compassionate/humanitarian grounds what are the chances of success and will they be required to stay in Canada during the time when their application would be processed? I do pray for some guidance from this forum