Well, that's one more of Bioware's failures while writing this part... I didn't tell anyone about Quinns betrayal but somehow Pierce knows!!!
I was with (lightside) Jaesa in the transponder room, and I chose the option "your secret is safe with us", and Jaesa said "it is for the best"
then, I was slow at raising my affection with Pierce because I don't really like the guy. what was my surprise when I got my third conversation with him on corellia and he told me:
"know I've said it before. but I kept my mouth shut a while now. Time to speak up. Quinn betrayed us once. Long as he's alive, he could do it again. Puts the whole crew at risk"
How on earth is he supposed to know???
After that you get several options, you can say:
" I rely on you to inform me of anything suspicious." + 40 affection with Pierce and he says " Good, Quinn won't go to the fresher without me knowing"
or SW: "ask me to kill Quinn again and I might kill you instead"
My personnal favorite but -81 affection with Pierce
So I finally went for the middle way" You may hate Quinn, but you and the crew are perfectly safe", and then Pierce answered " Hard to serve with a man you don't trust".