Nothing in the ssecurity settings so that can't be it. Maybe she can try to log in here on the Support board and then go to your board and see if she is still logged in. The only thing I can think of is maybe she has the date and time wrong on her computer. It's easy enough to change that and it would keep her from being able to log in. So ask her to check that, if it's set right then she should come here and try to log in. It might be easier for her to log in here and then go back to your board since you have Membership Screening enabled that is what I think will get her back in. Being logged in before going to the board.
If that doesn't do it we need to know what error message she's getting when trying to log in here...Because that message she's gotten sounds like it's related to the membership screening.