I have BPD, and I often TELL people how to deal with me, lol. The key, for me, at least, is to NOT use "tough love". Now, when I say this, I do not mean just take everything or not be assertive. Often, people have good intentions, but they word things in a way which upsets me. The key seems to be rewording what you are saying with some sort of truth (NOT harsh truth, just say it compassionately) along with validation of feedings and sympathy. So, you may say something like:
"I can't believe you are being such an annoying little brat. Everyone can't have your attention all the time! I have things to do too, you know. And you never take in account MY feelings!"
You can change that to:
"Mary, I acknowledge you feel extremely lonely right now, and that must be painful to deal with. However, I am in the middle of something right now, but will be available shortly or at _________ time. Do you mind waiting for me, so I can dedicate that time fully to you?"
To be honest, I understand how you feel. My mom has BPD, but she is extremely abusive. The key is a compromise, and the BPD sufferer also has to take responsibility for actions no matter how painful, good intended, or confused they are. For example, my mother may be mostly responsible for causing me to have BPD, but I refuse to let people believe any verbal abuse I thrust upon them is THEIR fault, because they don't deserve that. I have found that when people reword what they say as I said, I actually realize my own feelings, and I come down from my anger and black and white thinking. The next step is to set boundaries, but explain why, and talk about them. Putting boundaries up one day can lead someone with BPD to feel imagined abandonment. The key really is communication. Encourage your friend to TELL you her emotions, so you can help them. I know it's hard, but there really is a sort of logic to why we do what we do. We tend to be sensitive, so something being perceived as too harsh sets up the wheels for abandonment issues, which causes black and white thinking, then anger, then abuse, then possibly "manipulation" to some in order to keep the person they are afraid will leave them because they are defective. Good luck!