She stole parts of the money she and her girls "earned" and blamed it on Rio.
I'm not totally sure if I've missed some important information while playing the game. English is not my language, so I may have misunderstood something after all.
I think you are referring here to the conversation Riona and Fuckface have in the latter's room in episode 9. This is the excerpt of that conversation that mostly covers that topic:
Riona "I asked him (Tommy) if we could share a smoke - a blunt, you know?"
Riona "And he told me I've been smoking for free. But I never did, I've always paid for myself."
MC "Did you smoke his stuff a lot in the past or...?"
Riona "It wasn't about his pot, it was in general. He said that Quinn told him..."
MC "Oh? So, she's lying?"
If the MC discussed the Cathy's cluck with Rio in episode 4 and then confronted her at the Alpha Nu Omega party in episode 5:
Riona "Yeah, she is. Remember when I told you that we had problems with money?"
MC "Yes."
Riona "I think Quinn is blaming me for it by telling Tommy I smoked for free..."
Under other circumstances:
Riona "Yeah, she is. Don't tell anyone, but we had problems with money some weeks ago."
MC "Who are we?"
Riona "Me and Quinn."
MC "I'm a bit confused. What kind of problems?"
Riona "Just money problems."
Riona "And I think Quinn is blaming me for it by telling Tommy I smoked for free..."
Riona "...and possibly...that it was her who actually smoked for free."
Riona "Sorry...but I can't tell you more than that."
You can take Riona's suspicion as a proven fact... or not. After all, it is not as if Riona is not growing upset towards Quinn, is it? Or as if she also likes drugging herself a whole lot, you know...
Anyway, even if Quinn had actually stolen money, my opinion still would stand the same. I still couldn't see her being beyond redemption.