J.C.
Whoa - why are they continuing to let her work? If they wanted to fire her for misconduct then they needed to do it on the spot. Not let her continue to work. By continuing to let her work, they deemed her trustworthy enough to do so.
She shouldn't "let" them fire her at 3:30. She needs to speak up for herself. She needs to ask to be put on a written warning (or a final written warning saying that if she breaks the rules again, then she will be fired on the spot).
Good luck!!