This is a tough decision, think the real question is what can you do.
You can attempt to help your friend who lives far away, tell people who can do little to help her or you can ignore her and let something potentially bad happen. I think this is one of those decisions where you have to allow it's natural course to take place.
She's your friend. You have a responsibility to help her, and in this case, save her. If she doesn't listen to you that it's wrong, yes, go tell her family. Why don't you tell her why what she's doing is wrong? The effects and all. Tell her what happens if she keeps doing that stuff. Maybe she'll listen. Don't stop telling her it's wrong. Don't ever. Good luck to you on this. Don't give up, even if your bones break.
You could ninja-douchebag secretly tell her family and hope they're not stupid enough to be in denial and ignore you or worse rat you out.
If she's abusing prescribed drugs and mixing those with alcohol or other drugs she could be dead at any time. Prescription drug abuse is racking up a huge body count every year in the United States. Less people are dying in Syria because of war then fools who abuse prescribed medications in the US.
It's probably an issue for many nations.
If anything, you're still her friend and keep that in her mind as you gently attempt to get her to rehab. Just don't throw it at her like a hammer. You need to be a Jedi on this. There's almost no chance she'll quit cold on her own, that only happens when you've been 2 seconds from death and if you have to wait for that to happen you might as well get ready for a funeral.
Do you even know what pills she's taking and what they do? Calm down. Personally I would do nothing, it's her life. Also, I would consider it a betrayal of trust if you did do anything. I would also lose faith in you as a friend if you started freaking out like you're doing.
As omega pointed out, what you're saying is really like letting someone put a rope around their neck. Look at this girl's avatar, she looks 16ish, how old do you think her friends are likely to be? If kids could be trusted with their own lives they'd have freedom to do a whole lot of things sooner than 18.