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 parent adjusting to an empty nest, use this factual base: acknowledge the event and practical strain using only information the parent has chosen to share.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer a meal, transport, childcare, advocacy, ordinary company, or another specific manageable help. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not offer diagnoses, compare children, demand updates, or post about the family. Use medical, safeguarding, crisis, domestic abuse, or emergency services when a parent or child may be in danger. You do not need the other person to agree with a boundary for you to follow it consistently.