To put it more simply, once you've met Marble in the cave, there's a daily check for where Trevor will set up. Basically, Trevor has 3 spots he can set up: The Tavern, Qhala's Brothel, and Marble's cave, and every day the game rolls to figure out where he'll be that day, skipping any spots you haven't been already (so if you haven't been to the Stone Knife yet, it'll choose between Marble's cave and the Tavern, and vice versa). If the game puts him in the cave, then you get the DriderP scene as soon as you enter the room. If not, wait or rest until the next day. You don't ever have to so much as buy a single item from either of them.
Or, in other words, it's (not very) random whether or not you get the scene! (I guess you could savescum it, if you wanted, though just playing normally seems faster?)