I am so very very sorry, how painful this must be for you and her family as well!
My grandfather took his own life years ago. I was too little to remember much of that, although I do remember him. The house was full of people; his kids and grandkids. Nobody understood what was about to happen. They actually knew he was depressed, too, but I think it's difficult to immagine that a person can do something like this. As they all look back, they remember that he went in privat, to all of his 5 kids, one at a time, hugged them tight and said he loved them (he never hugged and said "I love you"). They were all surprised by this, but never saw it as a good-buy until after he left and died. That's when they understood, but of course it was too late. It was extremely hard on the whole family. But my point is, sometimes there can be pretty clear signs and people will still not really see it until it's already happened.
This is very tragic, what happened to Rachel, but you really must not blame yourself for it. I know it's a very natural thing to do, but this is not your fault, nor is it anyone else's. Us humans are very good at seeing things in retrospect, but it doesn't help anybody.
I am so sorry you're going through this. I feel very sad on your behalf, as well as her family.