Can a Green Card holder stay overseas more than 6 months

[atjubair]

Hi!

My mother is a green card holder and she left for Bangladesh on November 2014 at this time she is staying in Bangladesh.
My mother wants to stay in Bangladesh more than 9 months, possible 14 months.

Can she stay in Bangladesh for 9 to 14 months?
If Yes, what is process to get proper papers for her re-entry.

Thank you very much for your help in advance.

Regards,
Ahmed