Do not force them to change otherwise they will against you. Most human/people will change based on their real life experiences. But, you can remind them at least 2-3 times only then stop forcing them to change.
If they didn't want to change, believe it them. Believe that they won't change at all. Then, let them explore it self. They learn their mistake after facing the real situation. Is enough to say "Ok, I let you do, but if you need help, I always be there for you" then you just go - trust me they will back to you later or anytime soonest.
Whenever he/she express their feeling frustration and they need someone to talk too, be good listening but in the same times, you can add some advise in right way. DO NOT COMMENT TO CONDEMN OTHER PEOPLE.
After give some advise, give also some idea to improve them too. Since they already make some mistake that remind them entire their life. So, support with great idea to improve. Give also some morale of the story but yeah it can make them more regret and remember.
Tips for you:
Hope it workable to you.