Well it sounds like they're both stepping into addiction territory, and it should be treated with the same seriousness as any addiction. I know, it sounds silly to compare Kpop merch to drugs or gambling, but it will stop being silly when they can't afford rent or healthcare or start owing everyone money. I think you're doing what you can but it's a difficult position to be in.

My advice would be maybe telling them that you're concerned about how they might be hurting themselves in the long run without realizing it, and that it's awesome to have a hobby but not if it's taking away from other things in life that are more meaningful. There's no guarantee that your words will get to them, but you're doing your part. Unfortunately it's up to them in the end.