Yeah, I can totally see the "scared shitless, I don't want to do anything to antagonise Katrina" part, but that scene was definitely a bit odd. Would have been better in terms of believability if he didn't take Tris into the bathroom for some fun first, even if that did give us another lewd with her. He should have been absolutely freaking out - "I can't explain right now, but you have to trust me on this" sort of pleading for her to get out of there.
I think RC could have added a few images and changed the tone a bit to still get the lewd scene and have an explanation for why Tris ignored the MC when he was trying to stop her/explain that this was a bad idea, at least in the club.
When she kisses the MC, he detects a strong chocolate taste in the kiss, but he misses her, so he ignores it.
Triss tells the MC that she is So horny for him and she Really needs this, interrupting him every time he tries to talk her out of it.
After they have sex, her clutch falls to the floor and out spills a small box of chocolate candies - of the type the MC knows very well.
Tris calmly collects the candy box, telling the MC that Katrina gave her some as a treat for her first week and don't they taste yummy, plopping one in his mouth before he can wrap his mind around what Katrina has done to Tris.
This would create more of a reason for public sex in the club - something the MC has to distrust (even on the Darlene path) and set a tone of urgency for getting his mother and sister to avoid Katrina at all costs - with the MC having to tell them more about his encounter with her so that they believe him.
His unwillingness to tell them about how dangerous Katrina is, and even a little about how he knows this, now puts more of his family in the path of danger.
Cheers!!