Ask them directly. “Are you thinking of ending your life?”
Almost every family hesitates here, because of a fear that asking will plant the idea. It will not. The evidence on this is consistent, and it is the opposite of what most people expect — asking plainly tends to bring relief, because the person has usually been carrying it alone and waiting for someone to notice.
Ask it in the words above. Not “you’re not thinking of doing anything stupid, are you” — that is a question with only one permitted answer, and they will give it to you.
Then, mostly, listen.
Don’t leave them alone. Not to make a phone call, not to fetch someone. Take the phone with you into the room.
Quietly reduce what is within reach. You don’t need to search the house or announce what you’re doing. Move what could be used to cause harm somewhere else and keep hold of it yourself. Of everything on this page, this is the step that does the most.
Call 14416, with them if they’ll allow it. You can call on someone else’s behalf. You do not need their permission to ask for advice about how to help them.