Maternity Leave “resignation Acceptance” Letter Without Resignation

I work in a large casino. One of my coworkers recently left for her maternity leave. Before that, our employer continuously denied her accommodations like being allowed to sit/ bullied her to stand and chastised her for taking too many breaks. She ended up having to go on leave earlier than intended due to life threatening complications. Our boss made comments about replacing her and another pregnant employee as well.
A while after having her healthy baby, she was ready and excited to come back, but 2 days before the end of her leave she received an email from our employer saying they accepted her resignation. She obviously never resigned as she was gearing up and looking forward to coming back. They will not let her come back.
This feels illegal. Is it?
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