Never, under any circumstances. Its too easy to get sucked in when you're the good object, and to be seriously physically, emotionally, or financially hurt by someone with diagnostically poor boundaries and a predilection towards impulsivity. Given the enormous overlap/ causal relationship between BPD and past abuse, its easy to trigger or get caught up in the crossfire between the person with BPD and her pasts relationships and perpetrators of abuse. Life is too short to play with fire, or to confuse new relationship energy with love, while dating someone with chronically poor abilities to distinguish the two.

You won't know when your BPD Sb has been triggered, gone off her meds, resumed self-harm, started drinking/ drug abusing, or otherwise fallen into destructive patterns until you have been burned by an explosion that she is used to walking away from, and you are not.

TL; DR: Fuck no.