Thank you for the additional information!
The amount of emotion she is displaying is out of proportion to the situation. It's not as if you are getting married next month. Crying over a simple email is not a normal reaction, so then the question becomes what else is going on?
I think its interesting that things got worse after she visited her parents. I think there may be more to that coincidence than their unfortunate statement about not having any fears. I can make some guesses as to what else there might be, but they are only guesses, of course...
People who grow up to be control freaks often were raised in chaotic homes, and the chaos could be due to alcoholism, mental health problems left untreated, domestic violence or family secrets. Her desire for a guarantee for future happiness is unrealistic, and may indicate that she has had to deal with "shifting sands" (feeling like nothing is stable) in her family of origin.
If this were my friend, I would help her find a good, supportive therapist to talk things out with. This much crying and worry may be the beginnings of a serious depression. The fact that it's affecting both her job and her relationship mean its time to get serious help.
This is treatable, and she can get past this. Try to be as patient and supportive as you can in the meantime, as you sound like you have been.
I wish you both the best,