Make room for the other version
Ask what the other person understood before assuming that you are arguing about the same facts. For a message about how to support someone grieving an in-law, use this factual base: use the person's name and relationship when known, and avoid details the family has not shared publicly.
Correct the record precisely
Correct only what you can support, acknowledge any real impact, and propose a practical way to prevent repetition. 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.
Agree on a testable next step
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. Do not recruit an audience, publish private evidence, or turn a misunderstanding into a campaign about character.