They just need to realize this on their own or in therapy if they can stick it out. I just don't think you can "solve" it for them by treating them a certain way
Cleaningguy, I totally agree. Most of the time I was with my ex-HPD I instinctively tried to give her stability and not to fall for her childish games, her drama or the "shallow", manipulative, cold type of seduction. However, trying to maintain a romantic relationship this way, and putting a lot of energy into staying detached and alert, simply doesn´t make sense. Eventually, either you fall for the seduction and get dumped sooner or later, or you leave the scene by yourself.
A therapist with therapeutic tools in a professional setting can stay detached and alert and is paid for this effort. But even then you can´t be sure the HPD can be cured.
So, simply providing stability and validation in a relationship won´t do it. It needs therapy, and still the outcome remains uncertain.