There really isn't enough context in there (what's an angry letter? To who(m), if anyone? What is the plan for sending it and when (like after a 2nd draft?) What is the purpose of the letter? What does "b****", if that is the word used, mean to him?

Once I was doing my lawyer thing, and ran into a bureaucrat both lazy and stupid and suggested she do her job in haste and correctly. I was not rude in words or tone. Later she saw my client by herself and said, "your lawyer is a real b****." My client said thank you, and I was pleased.

In this context, b**** meant someone who was an advocate for another, who accepted no S***, and who got the job done. Some people use this as a proud beacon for women who are "assertive" if they were men, or who otherwise refuse to accept poor treatment from anyone.