I doubt that anyone here will have been in a similar situation (or know someone who has), so it's probably unlikely that anyone can provide you with a definitive answer.
As I read the US government's official advice online though, it appears to me that your friend might need to make an appointment to visit the US Embassy in London, in order to obtain a 'Transportation Boarding foil'. The process itself would seem to only take a few days but, as the document is only valid for 30 days from issue, it's a little too early for her to do it yet:
https://uk.usembassy.gov/lost-or-stolen-green-cards/
If she's not already done so, she should contact the US Consulate in Belfast for guidance:
https://uk.usembassy.gov/embassy-consulates/belfast/us-citizen-services/