What do I need to do to have my mom to come back US for a visit when she has been out of US for 2 years?

She has a US Green Card and it will not expired until 2013. The issue is that she has been gone living in Canton, China for more than 2 years now. What I understand is that you can not be outside US for more than 6 months for a US Green Card Holder. Now she wants to come back for a month to see her grand-daughter. Is there a form to fill-out so she could come back for a short visit without any issue at the immigration? What do we need to do?