It sucks but, by her own admission, the job worked fine for her before she became pregnant. You're in a position where they are trying to wring out every bit of efficiency because from most big businesses perspective you are a cost that is hurting their shareholders.
I'm not sure why they needed a doctors note for her to clock out to use the restroom. Seems unnecessary
They probably fired her for using her FMLA which is screwed up but that's what huge corps do. She became an employee who was more costly than somebody they could replace her with
I think unions tend to take things too far but at will is pretty messed up imo. We had a guy take a contract job for us a couple of years ago. He'd been with a company for 8 years and one day they dismissed him with no reason. He asked but they wouldn't give him anything. Almost a decade of his life and they couldn't even bother to give him a reason or any closure.