If she's going to get addicted, she'll get addicted. She makes the choice to put the coke up her nose (or smoke it). I know you don't want to watch a descent into addiction, but if it happens, that's her choice. You probably just need to look in another direction.

Remember you heard this here first. Don't loan or give her any money if things shake out like you're worried they will. You won't ever see it again, and then you'll be mad both because of the I-told-you-so addiction factor and the fact she took your money.

Is there a way you could get a healthy girlfriend to take your mind off this situation? Then at least when you and the healthy one break up later and she starts yanking your chain, you'll be being yanked by someone who's healthier than this chick???