A problem or not??

Hi all,
A mate of mine is flying to Edinburgh on Sunday and seems to be under the impression that she needs a passport. However, when I flew a domestic flight with BMI a few years back, I did not need to show my passport. I know my uncle is a CIO at T1, I can’t get hold of him!!

My question is:

Does she need a passport or not for the journey (she actually has an out of date passport) or is she kinda screwed.

Thanks in advance