I felt the Mona situation was relatively straight forward. She signed up with Quinn to fuck for free tuition, but balked when she found out she'd be fucking old guys:
Quinn: "You two. Leave."
Lily: "No. We're staying."
Quinn: "No, you're not. It's girl talk time."
Lily: "You're in luck! I'm a girl."
Lily: "Also, I already know what it's about."
Quinn: "..."
Lily: "Don't worry. I keep secrets very well."
Quinn: "Hopefully better than others."
Mona: "I wasn't going to tell anyone, she overheard us talking."
Quinn: "And \"us\" alludes to you and Camila?"
Mona: "I'm sorry, I couldn't do it."
Mona: "I thought I could, but as soon as he started touching me I felt sick."
Quinn: "Pathetic. This isn't the same go-getter I talked to before."
Quinn: "You know what you agreed to."
Mona: "But I don't want it anymore. You never told me I had to do old guys!"
Quinn: "Mona, Mona, Mona..."
Quinn: "What do you expect me to do now?"
Mona: "I won't tell anyone."
Quinn: "That's a promise that means nothing to me."
Quinn: "Case in point, Lily."
Mona: "It won't happen again. I promise!"
Mona: "I won't say another word about it or the drug deals."
Lily: "The drug deals?"
Mona: "Oh..."
Quinn: "..."
Why she leaves we don't know. She was paying her own way initially, then got the "scholarship", then the "scholarship" was rescinded, then she quit college. Probably wanted to put a very uncomfortable experience behind her.