Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to support someone returning from rehabilitation, use this factual base: use the person's own language for recovery, name the practical concern, and distinguish a lapse from what you do not know.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer a specific sober or practical support option and encourage connection with qualified help. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not shame, bargain, lend money without limits, or make your relationship the reward for recovery. For overdose, dangerous withdrawal, suicidal intent, violence, or immediate risk, contact emergency or crisis services now. You do not need the other person to agree with a boundary for you to follow it consistently.