After filing I-751, while in the US, she'll receive I-797 letter (receipt, Notice of Action, extension letter - people call it many ways). It takes anywhere between 4 to 10 weeks to receive the letter in the mail. You also get digital copy in account, but original watermarked letter is needed for travel.
Then her GC expiry will be extended by 48 months (4 years).
She can then travel internationally with valid passport, expired green card and valid original extension letter.