Answer:
I would suggest, instead of writing on a letter, you should really talk to the person in person. A letter could make them angry but that will be up to you. You can't make a person change and if they don't, it might be best for you to end the friendship instead of trying to fit the puzzle pieces back. Understand that you need to think about your feelings and why you want them to change. You should ask them questions about how they feel and what they are thinking. It will take baby steps.
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Here is an outline of what I would write:
Dear (friends name),
(tab)I'm am writing this letter because I am greatly concerned for you. I'm not happy that you are (doing whatever he did| example: being suspended from school), the person I met wasn't like that back in (grade level). I remember the (nice things about that person you like| example: nice) person I met. You have changed a lot and it's concerning that you are doing all these (bad, terrible, rude, mean) things.
(Tab)As a friend, I am willing to help you make a fresh start. It's time to see my (friends name) and not this (rude, adjectives like that) (name). I want what's best for you and I know this isn't your best self.
If you can't change, I guess we shouldn't be friends.
Sincerly your dear friend,
(your name)
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In all honesty, some people will not change and if that is shown, it might be better to stop being friends with them. It's really sad but it's the truth. There are some people who change for the worse and don't come out of the phase. By that time, it would signalify that you would need to walk away from that friend even if its hard.
I am sorry you are going through this. Stay strong.