It’s your right to try and act as a counselor to them, but it’s also your right not to. If you are emotionally invested in their welfare for personal reasons, that will absolutely skew your priorities away from shunning. So will the sunk cost fallacy.
The hardest thing to do is to protect yourself when those circumstances are combined, and someone is this close to getting over the hump, and it’s infinitely worse when they’re family/friend. I’m sorry you had to live through that, no matter what you chose.
I have done both things at different times with different people. Lately, after some recent experiences, I err strongly on the side of letting go of myself, abandoning any pretense, and trying to be unconditionally supportive. I would make some attempt to do that even if I were not especially close to the person. A lot of people tell me they don't feel comfortable extending themselves in that way and that is fine, but I have decided it is the ideal we should aspire to.
The hardest thing to do is to protect yourself when those circumstances are combined, and someone is this close to getting over the hump, and it’s infinitely worse when they’re family/friend. I’m sorry you had to live through that, no matter what you chose.