She is not being a friend to you, she is taking advantage of your friendship and she probably realizes you won't push for the money or trailer.
Times are tough, but you also have to live and feed your horses so times are tough for you also. You need to be direct and to the point and if she gets mad and you lose your friendship, then it seems is must have been onsided, you concerned about the friendship more than her.
If she cant pay the $50, then she should not have a horse is the way I see it. I bet she drives her car which requires gas, has a cell phone which requires money and does what she wants, doesn't she?
As far as the trailer, her brother and girlfriend need to vacate it or pay the $200 immediately. They are also using you and using the "we are friends" thing with his sister to keep living in the trailer.
A friendship is a wonderful thing, but sometimes when one person takes advantage of it, its not longer a friendship, its based on how far she can push you to confronting her.. Tomorrow is the last day of the month, I say she needs to pay you what she owes you or you need to forget the friendship and get your money any way you have to.