You don't address her being in her underwear in Tommy's room. Being in her underwear in her own room is far different to that.Is it supposed to prove anything except that he took hard drugs ?Do you assume she also banged Dawe in Ep3 cliffhanger because she was almost naked as well ?
Tommy's attitude during the whole game makes clear he has a crush on her but never has her way with her. Knowing the man he would have bragged about it, instead he's just stunned when he learned that Tremolo fucked her on the roof. Besides if you can find a single scene where Quinn seems to show any interest in him from a sexual or romantic point of view ?
It makes sense with his character and his absolute hatred of Vinny. He never was the alpha male and was always overshadowed by Vinny, then the MC who both did what he couldn't.
There is no reason for that if she is just giving him drugs.
Either:
1. She is in her underwear because they are having sex.
2. DPC wants us to think they are having sex, for some reason.
Why else make the scene and put her in just underwear?