Kay, I know that Vick is just breaking your heart. Maybe Ninnie is right and she would sell to someone else.
If that doesn't work, well.....I'm assuming that 50 lbs on a mini or shetland is more like 200 on a full sized horse. If all else fails, you might try contacting the local SPCA or Humane Society and making a report of an underweight horse. They will do an investigation and speak with and work with the owner to educate them and remedy the problem. If they don't comply, then the agency will seize the horse and you can get him back from them.
Can you tell I watch WAY too much Animal Cops and Animal Precinct? It drives Mom nuts, 'cause I'll call her boo-hooing about some really pitiful animal.