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 a family while someone is in intensive care, use this factual base: acknowledge the diagnosis or treatment only as they described it and remember the practical burden around it.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer one specific task, flexible company, or ordinary contact that they can accept or ignore. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not prescribe cures, compare outcomes, ask for optimism, or share their health information. Leave medical decisions to clinicians and contact urgent or emergency services when there is immediate danger. You do not need the other person to agree with a boundary for you to follow it consistently.