We can only love them and be honest with them about the way we feel. We can be present, and helpful, but we cannot allow ourselves to be used as fodder for their fire. We are worth more than that.
It's a hard thing to realize that the best we can do when others are in reckless resentment and self-abuse mode is to take care of ourselves. It always feels a little bit as if we should fall apart with them. But that's not the answer- not for them, or us, or anyone. It's live and let live that's required, with our hearts open, and our self-care boundaries intact.
I take care of myself first and foremost because that's my main job. I am loving towards others, but do not throw myself under their train out of a sense of impetuous co-dependence.