The Uni probably won't tell you.
It's more likely that she got a 3rd than anything else. Perhaps if she failed an exam/essay and could only get a 3rd (or even just passed without a classifcation, do they still do that?). If she tells you, would you start questioning it, asking but why did you only get a 3rd when you got X grade on your dissertation... then she'd have to tell you about missing a deadline or failing an exam meaning she can't get above a certain grade and then deal with you being all disappointed, or even having a go at her about it.
Not wanting to go to her graduation suggests she's fucked up the exams and is annoyed at herself about the whole thing. She's not proud of what she's achieved. Or that she's not actually going to graduate this time round, one close family friend failed some of his final year exams, he passed everything else and so had a choice, resit the whole year, or go back and just do those exams the following January - he had a job lined up who were happy to take him anyway for a Sept start, so did that.
Leave it, don't try and be sneaky, because what will you do with the information once you've got it? Confront her? Tell her off for the poor grade? Keep it to yourself so you can say "I know" when she finally tells you? What will you do with the information?
(Assume it's bad, if it was a 1st, she'd tell you)