Yeah, the good Jack path is probably my favorite outcome for Ash. He turns out to be a really nice guy, and you can absolutely have Ash be a good girl and they end up in a healthy relationship. The easiest way to get there is pick up enough bad points early that he sends her a dick pick when they first start messaging each other, I think you need 5. You want him to hit on her rather than Eva when they go to his gym together. After that, like
msleomac said, just always pick blue options with him, show interest in him, and avoid the gang bangers and don't do drugs when Jess or Jack offers them. I also typically have her get pissed and break up with Eric when he announces his plans to go to Somalia, but this isn't strictly necessary, it just allows her to get with Jack without cheating.