As @Rearviewmirror stated, do not file prior to the 90 day window opening, so look at the specific date and reference the link to determine the earliest filing date. She cannot do biometrics abroad to my knowledge, so if she gets requested to complete that, she may need to come back or try and reschedule with your local office. Also, if she is still abroad when filing, and her CGC expires, you will need to find a way to get the original extension letter to her as she needs the original to re-enter the US with her expired CGC. I also suggest making a few copies of the extension letter as sometimes airlines request to keep a copy.
Good Luck!
https://www.uscis.gov/forms/when-to-file-form-i-751