Your theory about the past would make sense. There have been times when Doug asked himself "when did she learn to do this?"Hmm, if Rafa is manipulating Doug like that, it's possible she was already cheating on him before the vacation without Doug knowing.
Maybe she was already a slut before Doug, who knows?
Unfortunately, we don't know much about the events before the vacation.
However, I don't think Bruce and Erick are real friends for Doug.
They often act more like his bullies from school.
And Bruce might be one of the reasons Stacy and Doug's relationship broke down.
The way Bruce came on to Rafa, it could have been the same with Stacy.
After her relationship with Doug, Stacy probably had an affair with Bruce for years, and as soon as she saw Doug again, she ended the affair because her feelings for Doug resurfaced.
If I assume that Bruce is at least partially responsible for Stacy and Doug's breakup, I could imagine him planning the same thing with Rafa.
Bruce seems jealous that women like Stacy and Rafa are drawn to Doug and not to him, even though he thinks that he is better looking and has money.
Doug really only has one true friend, and that's Jim. Jim is usually high, but he doesn't mess with Doug or make fun of him like Bruce and Erick do.
I think Doug is simply weak-willed; he can hardly stand up to Rafa, Bruce, and Erick.
I think towards the end, Doug will waver between love and hate for Rafa, and perhaps rekindled feelings for Stacy will also come into play, which won't make things any easier for Doug.
Erick and Bruce are definitely NOT his friend. I wouldn't be surprised if Erick stalked Doug and on the internet, found out about Raf and assisted Bruce in setting up this trip with intentions to steal Raf and humiliate Doug (just like old times).