I think of this quote a lot.

"This guy's walking down the street when he falls in a hole. The walls are so steep he can't get out. A doctor passes by and the guy shouts up, 'Hey you. Can you help me out?' The doctor writes a prescription, throws it down in the hole and moves on. Then a priest comes along and the guy shouts up, 'Father, I'm down in this hole can you help me out?' The priest writes out a prayer, throws it down in the hole and moves on. Then a friend walks by, 'Hey, Joe, it's me can you help me out?' And the friend jumps in the hole. Our guy says, 'Are you stupid? Now we're both down here.' The friend says, 'Yeah, but I've been down here before and I know the way out."

You've been where your friend is. That may allow you to sit where they are and just be with them. It's important to not be judgmental of the struggle and to acknowledge that suicidal thoughts can be valid, but also offer support and comfort to try to get them through the crisis. You can try to help connect them to professional help, and even offer to go with them to an appointment. You can talk about crisis resources, but as others have said you can also help them come up with a crisis or safety plan.

I will say that the time when I was lowest and closest to harming myself in the past few years, a friend approached me and asked, "Are you okay?" That felt really good when they asked, because I wasn't, and I had an opportunity to open up about that.

Whatever you do, make sure you're taking care of yourself too. Working with someone who is suicidal can be exhausting and it can also be triggering. Don't neglect your own mental health and recovery. Good luck!