I have been 15 with divorced parents, who had partners who were angry that I didn't fit in with the new set up.
I was fucking furious! My whole life had changed in a few short years. I had gone from being a loved child who felt secure to someone who felt unloved (not true-but my perception at the time), and was expected to fit in with the adults plans constantly, they had very little perception of my feelings other than I was 'difficult'.
I was powerless and full of rage.
It took another 15 years for my relationship with my father to get back on track.
Whilst a teen I drank, smoked, demanded money, well did all kinds of things to get some attention. I should have worn a t shirt with LOOK AT ME! On it.
So yes, now I'm an adult I can see that I was a royal pita. But, I also see I was upset and angry and what I needed most was time alone with my parents (separately), understanding, reassurance and kindness. Of course I made this very difficult.
I don't think a slap was the right reaction. I've seen my own dd1 whose 15 get beside herself with rage. I stop the arguement and hug her. She has a cry and often we then calmly chat over the incident before. I do this because I remember the fury I often felt at that age.
I think a request for money is to remind you both that she is still there. Her father should say it can be discussed when he next sees her.
I don't think she's behaving well, but I do think the adults need to behave better.