Are these real friends?

I think that you should try to get Mandy to open up about her problems and talk to you about them. If she wont open up to you than I think you should let her go and find a new friend. I don't believe that Stephanie is that bad of a person herself from your description. I think that the incident at your house was simply Stephanie being influenced by the way the Mandy was acting. So all in all I think you should try to talk to Mandy and you should keep Stephanie as a friend.