I'm sorry to hear about your friend.
I've had a simular problem with a friend. Honestly I think your on the right start by being there for her and looking out for her. To help my friend it was alittle easier since her parents were alittle more understanding. If your friend is worried about the number showing on her phone bill, has she tried using a pay phone or public phone. Also is there any places in your city that has a free help center for teens that could be helpful as well.
Once again I'm sorry not to be much help. It's just these situations are always hards since very person handles things differently.
I hope things work out for you and your friend !