Make the first sentence usable
Lead with the reason for writing so the recipient does not have to decode a long emotional introduction. For a message about how to respond when someone may be suicidal, use this factual base: reflect what the person has told you, any immediate concern, and the practical context without diagnosing them.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. 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.
Respect the boundary already visible
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. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.