Except that she doesn't know that. All she seems to know or suspect is that they each might have a thing for each other and this could be the reason why she likes messing with them becuase it's part sadistic pleasure in screwing with each of them but also part jealousy because she also likes him.
The thing about this scene is that it's not about the MC wanting to help because he wants to fuck her, which you could argue he shouldn't if he's with M&J, it's simply about offering help and Quinn refuses because of a stat check that doesn't align with what's happened between her and the MC.
Just for a moment, let's imagine this game didn't have a DIK/CHICK affinity system and moments with characters were simply based on the other moments you've had with them. Now, yes, in life you might have a "feeling" about someone, like an intuition that there's something you don't like about them which could be argued is what the affinityr system is all about, but how you view a person is more often based on your interactions with them, how they've spoken to you, how they've treated you, moments you've shared together, etc.
So, looking purely at that moment when he offers help through the lens of personal interactions between them, why would a MC who could have taken her number and used her services, been fucked by Quinn at the cum-petition, had some fun with Quinn on Rooster, been getting talked about so much by Quinn as to make Sage think she has a thing for him, fucked Quinn on the roof of the DIK mansion in what was quite a nice moment between them, and been congratulated by Quinn for passing Hell Week, be rejected by Quinn? Again, forget affinity for a moment, just based on these interactions, it would seem they have a good relationship, so why does she turn him down?
It would seem to me that, from a character standpoint, they are close enough that regardless of anything else he's done, particularly since Quinn doesn't know about it, that Quinn turning him doesn't seem like something she would do based everything up until that point that has transpired between them that would suggest she would be receptive to his help.
It would seem that this management system is going to be around for at least the rest of this season as it's not possible to do everything in this episode, especially getting enough money to repair the windows.
When your "best friend" leaves you without hesitant, she still stands for you ... and her best interests that being in the know. 
