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 grieving a cousin, use this factual base: use the person's name and relationship when known, and avoid details the family has not shared publicly.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer one concrete form of help and remove any expectation of an immediate response. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not say everything happens for a reason, demand strength, or center your own similar loss. For immediate risk, traumatic circumstances, or a person unable to stay safe, use local crisis or emergency support. You do not need the other person to agree with a boundary for you to follow it consistently.