To a certain extent, I did this too. I recommend trying to limit the number of times he checks on them. Try to limit it to 3-4 times a week, then two times, then reduce it gradually. It is easier to wean him off of the habit until he can stop.

Another thing he can do it distract himself and focus on his own happiness. I think his need for justice is influencing his actions. He can tell himself what I tell myself: even if they don't realize it, his abusers have punished themselves. When you hurt someone intentionally, you damage your soul. It's a mark against the soul. It's sort of like the spiritual version of, "I'm rubber you're glue. Whatever you do bounces off me and sticks to you." That's how I see it. Even if they look happy, they will always have damaged souls.