So, if either the conditional GC is valid, or if it's expired but they have the I-829 receipt notice that extended the GC validity (and it's still unexpired), or if they have an unexpired I-551 stamp, they would have valid documentation to board a flight to the US. If they return to the US, CBP might question why they spent so much time outside the US, but I don't believe they can deny entry. They might issue a NTA, and unless they sign a I-407, only an immigration judge can remove their status.