Hi, I'd like to help you with your question.
It is very difficult to not respond to people with personality disorders. By their very nature, they push your buttons and bring out feelings that are better left unexpressed.
Your response to your boyfriend's daughter is normal. So is wanting to protect him by saying something to stop the daughter. But you are right, this will not help and probably only make matters worse. Writing her a letter or talking to her will only provide her with more fuel to throw at the fire.
In order to help your boyfriend, you need to be supportive. It sounds like you are already doing this which is great. He needs someone who he can turn to for comfort and to talk with.
You can also help by providing your boyfriend with resources to help him learn more about dealing with his daughter's personality issue. This can serve two purposes, to help him understand her better and to provide him with answers on how to respond to her. Here are some resources to help:
Stop Walking on Eggshells: Taking Your Life Back When Someone You Care About Has Borderline Personality Disorder by Paul Mason MS and Randi Kreger
I Hate You--Don't Leave Me: Understanding the Borderline Personality by Jerold J. Kreisman and Hal Straus
You can find these books on Amazon.com or your local library may have them for you.
Your boyfriend may also benefit from support groups. Here is a link for him:
The support can help him cope with the stress and help him deal with how to talk with her and protect himself at the same time.
I hope this has helped you,
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