My personal take on karma/luck is that whatever kinds of choices you make will come back to you; not so much because of any cosmic arbitration, as much as how those decisions shape you as an individual.
If I keep screwing people over and taking the easy way out of things, that will make me a weaker person, and the more frequently I choose to behave that way, the higher likelihood there is that it will backfire. But that's not fate, that is just my own values beginning to atrophy into disrepair until even minor difficulties are too much to handle.
Same thing with luck: If I always assume that I'm lucky, chances are I will make a foolish mistake and pay dearly for it. But that doesn't mean my luck ran out; it never existed in the first place. Just because I didn't get killed the first dozen times I did something idiotic doesn't mean I'm invulnerable, and the more false confidence I feed myself, the greater the chance that I will run into tragedy.
This is a horrible thing to say, but if you really want to see someone crash and burn, make them feel invincible. Encourage them to do whatever stupid crap they won't shut up about. Stop dissuading them; let them learn the hard way. Most people won't listen to negative advice anyway; I never did. I had to make my own mistakes and nobody was going to stop me.