My American Girlfriend is visiting Sweden but was denied to stay the original duration. Can I appeal it?

Hey, we've been trying to find answers and none have come to fruition. My US girlfriend is visiting Sweden for the first time and while going through border patrol and customs she came across a problem where she could not confirm that she would have enough money for her stay. She works remotely and gains a salary and while it isn't much it's enough to be here comfortably.

Out of her two-month stay, she was only allowed to stay for a month and her passport was stamped with the date she crossed the border. Before her visit, she searched for what she needed to visit and there were mentions of her only needing her passport since her visit is less than 90 days.

She is staying with me in my apartment and I have plans to support her during her visit, we have tried to contact the US Embassy, Swedish immigration and the Border Patrol at the airport and we weren't given any answers as to how we can resolve this situation as my partner wants to stay for the period she already booked her flight to.

I was wondering if I could get any advice or guidance on what we can do for her. Thank you.