I understand it's very hard to see others going through this but as you've said it's having a negative affect on you, and you realise it's not good for you to be involved. You can offer suggestions for where to go for help, which I think you have done, and let her decide what to do. I hope she gets some help as well, but she needs to seek it from people who are stronger and not going to be affected by her behaviour. Being firm and saying no is ok.