If a green card holders needsto stay outside US for little over 6 months, what does he have to do so as to not lose the Green card status

Have evidence of a date specific return to the US. Try to return to the US as soon as possible and be able to explain the long time outside of the US; such as medical treatment, schooling, assisting ailing relatives, or other factors that the LPR had no control over. Be prepared to show the officer at the POE ties to the US, such as; tax filings, property, employment, bank accounts, financial ties, family, etc.

If planning on being out of the US like this again, consider filing the I-131 for a re-entry permit. It is valid for 2 years and is an affirmative statement to the USCIS and the CBP officer that the LPR does not intend to abandon PR. Good luck.