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 after psychiatric discharge, use this factual base: reflect what the person has told you, any immediate concern, and the practical context without diagnosing them.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer one manageable form of support and ask directly about safety when you have a serious reason to worry. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not shame, threaten, promise secrecy about imminent danger, or make yourself the only source of help. If someone may harm themselves or another person, contact local emergency or crisis services and a trusted nearby person now. You do not need the other person to agree with a boundary for you to follow it consistently.