While I'm sure Quinn would have nothing but contempt for Sylas as a person, I have to wonder what her stance is on mages. Is she blinded by the nobility's stigma toward them, or would she be more logical?
If Quinn were to learn that Lux was a mage, would she keep it a secret? Or would she be giving Garen some notice?
We don't really have that much in terms of characterization for Quinn so it's hard to say. My opinion is a bit bias since I love mages and main Quinn.
But I don't think Quinn would be anti-mage. Quinn didn't grow up as an elite, she grew up on the outskirts of Demacia in the wilder parts of the land. As we have seen in some of the short stories, the more rural Demacians don't hold the anti-magic view very highly. The entire one town was basically starting a mini civil war because one magical child was taken away from their town.
Quinn is also supposedly well traveled, if it hasn't been re-conned we know she has been to the Frejlord and most likely other places on scouting missions. She's also very solitary outside the company of Valor so I could see Quinn being an observational person and realize magic isn't what the Demacian government is claiming it to be.
I think the one argument that would have much validity for Quinn being anti-mage would have been her encounter with Lissandra if that wasn't retconned. Encountering such a fierce force of magic could potentially sway her mindset towards anti-mage or at the very least wary of their powers and how strong they can become which is a big part of Demacian's anti-mage propaganda.
She probably views them from pretty high up
Two Quinn threads within the same week?! This is getting out of hand!
I agree with Placentas. Quinn’s origins as a more humble/rural Demacian (as well as her introspective and free-spirited personality) means she probably doesn’t share the same dogmatic views towards magic as the city folk who live closer to Demacia’s center. That said, she’s probably wary of it all the same, having been raised to believe in Demacia’s virtues. I feel like she’d keep Lux’s secret if she knew about it, though it depends on whether she’s closer to Lux than she is loyal to Garen (and judging by her old journal entries, she isn’t the biggest fan of him).
It’d be cool to get an official opinion on this, though. Quinn desperately needs more characterization; her VO is quite old and doesn’t really share much about the character, tbh.
Like Placentas said i think she may be a bit more sympathetic coming from a more rural background and more traveled but I also think as a military woman she understands the necessity of keeping mages under wraps for the safety of the nation.
She's probably say, 
"Go for the eyes!"
