She would most likely have a problem returning to the United States if she does not try to renew it now. She is still a Lawful Permanent Resident of the United States, but she will be required to show proof of that status when attempting to enter the United States. Typically, Immigration officers at the various airports in the United States want to see the expired Green Card and a receipt notice for the application to renew the Green Card. She could also have her attorney in the United States take her passport to the local US CIS office following filing of the renewal application to have a stamp placed in the passport showing that she's still in Lawful Permanent Resident Status. All of this assumes that she is eligible for renewal, i.e., she did not violate Immigration law to the extent that she would now be inadmissible to the United States.