Please Help — Qestion on GreenCard

My parents had their green cards 7years ago. They left US 6years ago and did not come back due to health issue.  But they are ok to travel now. 
1.Can they come back with the same green cards they had 7years ago?
2. Do they need to provide any extra documents besides the green cards? Any trouble for them to reenter? 
3.Or the green cards are totally useless? they should apply for visitor visas?
Thank you so much for your help!