Okay so I feel like we need to contextualize a few things here.

Initially, she opened up to him about the acting teacher at her college leaving their job. Bojack almost immediately decided this was an offer for them to be closer together and decides to take that job without once consulting her to see if she’d be okay with her 50+ year old half brother who happens to be a celebrity suddenly being a prof at her college. Imagine if you found out someone like your mom or dad was a teacher at your school/college? It’s gonna be awkward as fuck and you’re gonna wanna keep your distance, right? That’s exactly what ended up happening. Bojack was being annoying and clingy and Hollyhock had enough at one point and decided to establish clear cut boundaries. Bojack complied. This is just the setup though.

We have the setup established. Now is where things start getting juicy. Hollyhock ends up running into Pete Repeat at a party, and finds out about how Bojack took some high schoolers to prom (something that’s already really awkward) and got them drunk. One of them ended up getting drunk enough to need to go to the hospital, and instead of taking responsibility for it, Bojack just left her there at the hospital with Pete to deal with it, an experience which was very traumatizing for both of them. Take this knowledge and combine it with her own experience of overdosing on amphetamines in his house (an event which was admittedly not Bojack’s fault), a connection has been made: young people get intoxicated around Bojack, often with substances provided by Bojack. Shortly after, the news breaks out about his involvement in Sarah Lynn’s death. We now have another person of a significantly younger age than Bojack who became intoxicated in his presence. Connections made, patterns established. This brings us back to the beginning now. This guy, a known drug addict, with a clear history of being around younger people when they end up being fatally intoxicated on drugs provided by him, now has a job where he’s an authority figure to younger people. It’s easy to see why she would want to keep further distance from him and potentially even cut him off.

Now, as for the nature of her disconnection from Bojack, I don’t think there was any better way to do it for her than by changing her phone number and sending a strongly worded letter to him. We’ve all seen firsthand that Bojack is a pretty good manipulator, and if he has the chance to verbally convince you to stick around, he will do it and he will succeed. He did it with PC. He did it with Todd. He did it with Diane. He did it with Wanda. All these characters would’ve ditched his ass if he didn’t sweet talk them into sticking around just a little longer. Hollyhock would’ve recognized this and decided that the only outcome that would result from telling him in person was that he would sweet talk her into backtracking on her decision to leave him. Ghosting him completely was the only way.

Tl;Dr: the relationship was already awkward when he decided to take the teaching job at her college without asking her about it. After that, the combination of her own experience, Pete’s testimony, and the knowledge that came out about Sarah Lynn, Hollyhock was able to conclude that there was a very concerning pattern involving young people overdosing on drugs/alcohol around Bojack that, in most scenarios, are provided by him. She also knows Bojack is a master manipulator and would find a way to keep her in his life if she talked things out with him. As a result, ghosting him was the only way to truly cut ties with him.