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 She was paying her own way initially, then got the "scholarship", then the "scholarship" was rescinded, then she quit college.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.
On a side note, I love how Quinn has no control of the situation here. Lily defies her twice and sticks around for the discussion, and then Mona blows it and mentions drugs too. It's fucking hilarious.