Write from facts rather than momentum
Pause long enough to remove accusations, assumptions, and details that belong to someone else. For a message about how to support someone with an eating disorder, use this factual base: reflect what the person has told you, any immediate concern, and the practical context without diagnosing them.
Offer a proportionate next step
Match the size of the request to the relationship and to what the recipient can reasonably decide now. 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.
Protect privacy on both sides
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. Use the most private legitimate channel available and do not share the exchange to recruit support or pressure.