Update:
I gave the letter to her today, and I am shocked at her reaction. I was watching her read the letter, and there was no reaction to it. She finished reading, and she blew me away.
I was expecting "WTF?! How dare you say this?!" but she actually said (along these lines) "Thank you for writing this letter. It shows something about your personality, and I really appreciate it. I fear the risk of losing you, and I can see a day where you could become the head (lead technician) here." I think to myself "Whoa...what was that?" That was something.
In the letter, I told her I was sure she wouldn't change, but she promised to change. In 30 days, we're going to "review" her behavior to see if she made any improvements.
I don't recommend you guys do what I did because you never know how the boss will react. If you're going to write a letter, make sure its not like mine...write something nice that shows concern about what he/she is doing/saying.
PS: I removed the fiance part because of couple of people telling me to remove it. It may have been the right thing to do.