I'm going to sound really wrong here but I don't understand why $50 matters? If the agreement was $150 and she paid u $100 after not actually staying a night, I would be pretty content with that. May explain why I've been walked over in the past.
Oprah once said and I agree that you should never lend money to friends. Only give what you can afford to lose. Maybe easy for Oprah but she has a point.
I would be very careful about your level of involvement with a friend so unstable. You don't have to disown her but you need to be careful her dramas don't affect you or bring you down with her.