When is it time to stop?

So we all know the saying, "You can only help someone if they are willing to help themselves." Would you apply this to everyone? Specifically those who are addicted to drugs? Or those who willing go back to someone who's abusing them? When is it time to stop trying to help them? Is there ever a time to call it quits? What if they refuse help? 

I know it must sound harsh, but at the same time, there's only so much you can do. I have a friend whom I've stuck with since the day I met her. 8 years later, I thought were we're still friends, but I don't know anymore. She disappears for months and then returns with some new guy. She's been through hell and back. She's done so many drugs both willingly and forcefully. I've recently spoken with her mom, and she told me that she never stops, that she only takes breaks and that she can't do it anymore. She said she's done because she can't put their family through that again. Every time she came home, it was with bruises. Every time she came home, I went to see her. Last time, it was because she needed surgery due to the fact that she was thrown out of a moving vehicle. That's the last time I saw her. Then she went back to the guy, back to drugs, back to everything she said she was done with. So when is it time to stop? Because I don't know how much of this I can keep seeing. She IS my friend, and I do love her, but she won't let anyone help her and she certainly won't help herself. We've offered her so many options, but she keeps going back. I don't want to give up on her, but it looks like that's the direction it's headed in.