In general, if your child is obsessing over something, it behooves you as the parent to investigate it. Which is why you’re here, so good on you. :D
Her fascination with the show may be completely unrelated to her troubles, it may be her reaching out. You won’t know either way until you look into it. All we as the fandom can really do is let you know that the show deals with the issues your daughter is facing, so there is a distinct possibility for that connection. The rest is up to you.
Prior to the Biscuits Braxby interview, Hollyhock’s behavior was reasonable at least. She didn’t handle Bojack’s imposition as well as she could have, but she’s young, Bojack is still relatively new to her life, and she already has the baggage of the weight loss pill incident. It wouldn’t be fair to expect her to be good at establishing boundaries with Bojack, who struggles with the same issue at the same time, just less destructively than normal thanks to the rehab. She was rude, and I don’t think Bojack deserved it, but I wouldn’t hold it against her, either. It can be looked past.
After the interview, her reaction was completely correct. We don’t know what was in the letter, but we don’t need to know. Whatever it was was long enough to require a second page. I imagine it took one of two tones: excoriating and hostile, or plain and definitive. I usually go with the latter in my mind. Either way, all we know is that it’s done, and Hollyhock has committed to it. She would be right to react either way to the revelations of the interview.