Alright OP I don't know much about the ins and outs of boundaries and personal relationships but as someone who has trained in suicide intervention I can tell you that she has thrown up enough common red flags for people who actually commit (plan, substance abuse, prior suicidal behaviour) that this needs to be taken seriously. From a crisis point of view ALONE this is the advice I give.

I don't know where you live but you should research if there is a mobile crisis program in your area. It is a public service with professionals designed to handle precisely this sort of thing. If such a program does not exist and you are truly wrapped up in boundaries with your wife then it falls to you to keep her safe FOR NOW. She obviously trusts you enough to involve you this much so here's what's important:

For those planning suicide, the first key step is that she confided. This means that some part of her wants to explore not-commiting, and this is what you must seize upon. She's told you her story, so what she needs right now is support. Don't try and convince her not to do it, that isn't your job right now. You have to make immediate safety the focus. If the next time you talk to her you can actively listen to her problems, empathize, offer help, explore other support systems, anything that can take her mind off of her plan, because usually the plan consumes the person wholly.

Now the last step is the most important part. You have to ask her what it will take to keep her safe for NOW. In this, we delay the plan which is the biggest enemy in this situation. This gives everyone more time to breathe and come up with a longer term solution. This is not the end of her recovery but that's why this is intervention and not counselling.

She may be stubborn, it may take a long time to get to the "safety for now" step, and in too many cases there isn't anything you can do outside of calling the police. It may just delay the inevitable. All we know is she confided in you and that's a dim ray of hope for the suicidal.

If any other person who has training with this can see I'm forgetting something please add if necessary. I'm at work and am only going off of memory and trying to post this quickly.

Good luck OP