You need not worry about it. She needs not file for green card extension in 6 months, 3 months or no month. Let her just wait for the outcome of the N400 process. Make her an inforpass and have her passport stamped with the temporary evidence of LPR status. That stamp is good for a year and as good as the card itself including for travel.
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As long as there is nothing wrong with her N-400 application she should be okay. It is best to consult an attorney.
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You should be fine. In Chicago where I practice, if your green card expired while the N-400 is pending, you can make an Infopass Appointment with USCIS, bring your N-400 receipt and current passport, and USCIS will stamp your passport as current proof of your lawful permanent resident status. USCIS usually gives a 6 to 12 month extension. You should consult an attorney in your area about this practice.