Oh, I am so sorry to hear that, Hawaiian. How awful. I don't want to say it would be easy to do that in the hospital, but it wouldn't be impossible. At night there's a minimum number of nurses around. Fortunately, it's also very quiet, and the staff checks on the patients regularly.
As Olivia said, your fiance is not herself. Mania destroys impulse control. You two have some very painful discussions ahead of you, but right now try and hold your head high.
Ask me about my Bipolar Disorder!