Actually, your in-law can come back on their green card.
The risk is that, upon their entry at US customs and border protection, they will very likely be questioned how long they have stayed overseas since last departure.
They cannot lie if they are asked. Then the CBP officer may determine that they have lost their permanent resident status.
BUT CBP officer cannot deny their entry into US if your in-laws refuse to agree with CBP officer's determination.
Then the CBP officer must allow them to enter US and your in-laws will see an immigration judge who will determine their fate.
However, if your in-law accepts the CBP officer's determination and signs the paperwork, then they lost their permanent resident status, period.
If they are not questioned by the CBP officer and are admitted, which is very unlikely. Then they are ok and re-entered US as permanent residents.