Frustrated babysitter

So my friends a young single mom to a 1 year old daughter. Her normal 2 sitters watch her throughout the week but neither of them can watch her on the weekends. I told her that I would because she needs the help and we’re friends. I told her not to worry about paying me because like I said, she’s a young mom on her own, she needs the money. And her daughter has always been fairly easy. But for some reason this weekend her daughter will NOT listen. She screams and cries and throws herself on the floor if I tell her no or take something from her, or simply for any reason she can find. She constantly throws things and hits me. She’s never acted like this before. I work in the morning along with school work on the weekends so my days are pretty busy. So when she comes over at 6 I’m exhausted. And then her daughter throws tantrums for 4-5 hours until she goes home. Idk what to do. I want to help her, shes one of my best friends and I know she needs the help. But god I’m not even getting paid to deal with her and all she’s doing is throwing fits. Im just so exhausted