I dated someone like this. Unfortunately, as great of a guy as he was, I ended up cutting the cord. There was too much back/forth and it seemed almost impossible to just take some steps back... he wouldn't have it that way.

You know, this might be a good question for the folks in the Personality Place forum... maybe getting advice from someone who actually HAS borderline would give a useful perspective on how you can work with your friend in a way that you're both comfortable with.

Good luck!
LMo