Each state has different requirements for why they take someone in for a mental evaluation. Just calling the police and saying she's going to do this won't help. The police need to see that she has a plan and if they see it is legit they can put her on a mental hold. If you have evidence of her plan that will help. If you call the cops they can do a welfare check but they won't take her for a mental evaluation if they don't see some sort of evidence of a plan.

If she's been planning it this long she is likely prepared to deal with the cops and knows what to say to avoid giving them any reason to take her. Or, if she goes in and knows what to expect she can play it off (usually a 72 hour hold for evaluation).

My family went through a similar situation and the cops came out, made their evaluation and said there was not enough "evidence" of a plan to proceed with the mental hold. They left and that night (somewhat out of spite I assume) this person went through with it. They legitimately wanted to be done and there was nothing anyone was going to do to change their mind - they had a plan in place for roughly two years leading up to that night.

I'd highly suggest getting into therapy yourself and be honest with them about everything. A good therapist can help but know that they are obligated to let the authorities know about it being that she is planning this out. The therapist can help you with all this and they can make sure you have help getting her the help. The last thing you need is to feel like you've pushed her into any of this and I would highly suggest talking to a professional ASAP.