It's tricky.
First off, I feel when one expresses they want to or are thinking of CTB, it tends to cause panic if they share this thought with a person who immediately wants to intervene in a "traditional way" ie call 911, have the person be brought to a hospital which the person very may well not want to go to.
Example: My brother. When our mother married her second husband, she acted like he didn't exist and treated him so, as stepfather 2 didn't particularly care for my brother. We only lived with him for a short time, but my brother had no bedroom - he slept on a concrete floor in the basement. He came to me one night and said he was going to kill himself and, his abuse of me aside, I knew that feeling and I felt he was going to do it. I saw in his eyes, he was terrified.
I made a call to my father who was in Michigan at the time. Now, it wouldn't have mattered if I had killed myself (I've discussed my fathers feelings towards me, his disgust) but my brother? No way. I told him of the conditions, the sleeping on concrete, etc., and a few days later I was looking at houses.
I was 15 - I either just got my drivers license or had a permit, I can't remember. Anyhow, I found a house for sale and and brought a friend along to drive by and check it out. The price was good, I made an appointment to view the house and the realtor said he'd never shown a house to someone my age - a first for him.
I called my father back in Michigan and told him of my find, and he basically bought it, sight unseen. He did come back and my brother and I moved into the house, and that was what helped my brother - having a bed, his own room, a fresh start in a way.
Example 2:
In my sisters case, after the birth of her first child, very soon after, she was going to attempt. She had gone to the bathroom and locked herself in - I don't know what she was going to do, but my niece began to cry in the crib and that sort of snapped my sister out of it and she came out to tend to her baby.
I think it was last summer when she had somewhat of a breakdown. She called me as she was driving and told me she had to get out and knowing how similar we are, I was concerned she was going to attempt. Now, had she been alone and attempted, or actually CTB, I would've been devastated though ultimately I would understand why. The issue was, she brought my 2 young nieces with her and that's when I got very scared. I was afraid she was going to possibly do something to them, my mind was racing.
She told me she didn't know where she was driving to, she just had to get out, and she was in a very dark place mentally. I made sure to stay in touch with her throughout but not so much to overwhelm her. It kind of felt like a hostage situation tbh as my nieces were with her and I didn't want anything to happen to them. She had brought a tent and was going to try and camp out somewhere but I sent her money to rent a cabin and that helped. They stayed there for maybe 2 days, and then she decided ok - she was going to go home. I was so relieved to know the girls were alright. It could've been a disaster.
Now, had my sister left alone, I still would've been concerned. Ending your life is one thing, but to take the kids with you? It was scary. I was able to remain calm when we talked, but after we'd hang up, I'd be crying. I would have respected her choice if it had come to CTB, but my nieces - I couldn't bear the thought of something happening to them.