returning from abroad

[immihelp]

I have a two friends who have green cards and social security and work in california legally. they want to go to vietnam to visit family what other paperwork do they need to be able to go there an come back to california so that there will be no problem till they are citizens. If you can please let me know that would be great.

[immihelp]

As long as they are traveling outside US for less than 6 months at a time, they don't require any other documents if they have valid green card and valid passport.