This is a excellent point. The 4th C contribute is equal to what other recovery programs like Smart call "our ability to influence". And we can influence recovery or contribute to recovery in both positive and negative ways. This is the first Ive heard of the 4th C in AA.
As for the letter, I went through treatment myself years ago and will say I feel the letter is fine. He will be hearing the same concepts about urges and dopamine while he learns about his addiction. If the letter does trigger him it can be positive because he can talk about it, and use it for positive learning on how to manage his thoughts and feelings. When he leaves the safe environment he will need to have defenses built up to protect himself.
Because of this, its my opinion the letter is appropriate and taking the time to write and let him know you are thinking of him, remembering past shared memories will be encouraging.
Originally Posted by
cynical one 
I’ve always kept my thoughts to myself on causing (it wouldn’t be too popular), but there’s a rarely talked about fourth C = contribute. And, yes we can and do.
In this case the son/brother is in very very early recovery. He has a long history of substance abuse and is now finally in treatment again. At this point in his recovery he is fragile physically, mentally, and emotionally. If he were to receive a manipulative letter such as this, it would most likely trigger a case of the *f*-its. From his mindset he is sacrificing everything to do the right thing, and now along with a long lecture on the evils of drugs, he’s being blamed for causing every dysfunction and health scare in the family. There is a time and a place (under the guidance of professionals) to do a debriefing. Now is far from the time. This letter made me want to go out and use, and I’m not an addict.