First of, it is BOUND.
Secondly, remove the tick from the die function and check after you lower the health. That is much cheaper.
Finally, you have to access the Widget and then call a function that updates the text. Then you just call the widget function as you would any class. The widget function simply takes the new value and convert it to text and then replaces the widget text variable with that.
HTH
UPDATE:
Example code.
The widget code. It initializes to 0 and then sets the new value every time it updates.
http://puu.sh/rbfja/127c8f74ae.png
The Blueprint code. I just used time, but the tick function would be replaced with your die event.