If they realize how long she has been away, they absolutely won't let her in. 5 years is way beyond any gray area that would allow them to use their discretion to let her in without an SB-1 visa.
She will either be sent back the same day, or held in an immigration detention center until they arrange to send her back. And the detention center is likely to be very far away so it may be difficult for you to see her. If she doesn't want to get locked up, she should sit and wait in her home country for a new green card.
They'll take away the green card for sure and she will have to reapply, but they won't let her back into the US (except for detention).
My mother seems to have gotten it in her head after "talking to some people" that they're just going to fine her a bit and let her in and maybe take away her green card so she has to re-apply.
EB3 ROW I-485 Approved: July 2007
USC: July 2013
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.