Reply

Mom questions are
1) whether this is the law that allows her to be able to come back to US with reentry permit even her GC expired ? 
-Yes, reentry permit means "Permit to reenter US" without other documents such as visa or green card. So, if you stay outside US for > one year, you have to use this document to reenter US, and the green card doesn't work no matter whether the green card expires or not.

2) since her GC will expire in one year, will homeland security will isse one year or two year re-entry permit?
-Two years.

Thanks a lot!