I didn't write that Zoey betrayed the MC. But now that I think about it, she actually did in a way. Was it stupid to listen to Bret? Absolutely. The betrayal is actually that she has known the MC for years and knows how he is emotionally. She's ignoring years of experience with the MC, who is her best friend, for advice from a guy she just met. In a way, that's a betrayal of friendship. But that's a subjective way of looking at it on my part.
The MC is not partly responsible for the breakup. To one he told Zoey to love her, only Zoey reacted shit to it. This left the MC confused and subsequently not knowing what love is. In EP10, the MC himself says to Zoey that he later realized what he had, because before he thought it was just the best friendship + sex. Zoey's reaction in the past is to blame for that. He did what a best friend should do. He let her go to San Diego so she could realize her dream.
Furthermore, we read so Zoey stopped responding to messages from the MC. The MC kept messaging Zoey and Zoey actively chose not to reply to the MC, her supposed best friend. The MC fought and wanted to maintain the friendship with Zoey at this distance. It doesn't matter what Zoey's motive was, you don't do that with your best friend. With that in mind, it's no wonder the MC said to Derek in EP9 that it felt like she threw him away. It's Zoey's fault because she's actually known the MC for years and knows how emotional he is.
Zoey runs away when things get tough, but Derek stays and helps. Neither of them are perfect. But I prefer friends who are loyal to the MC, even when things get tough. That's what makes a friendship in my eye.