I had a quilting friend that went to Paducah with me two years ago. She was a "gimme" friend. Always asking, always saying that she'd pay back and when it came time to do so would force a some piece of junk on me as pay back. Well, before we went to Paducah I told her, "Make sure you have your own money, I'm not buying lunch or anything else, I only have enough money for me." To make a long story short due to "dying for a drink" I handed her $4. She bought the biggest drink she could. On the way home I found out she had a bunch of money in her wallet. That was it, if she didn't value our friendship more than $4 then neither did I. I cut off all contact with her and suddenly my rulers, rotary cutter, books, fabric, etc. quit coming up missing.
Some people, no matter how much we like them, just aren't good friends to have. I'd do what everyone else does, charge her the $6000 PLUS interest, PLUS your bank fees, PLUS an "returned check" fee. If she didn't want to pay all that then I'd take back the horse and carriage and take her to small claims for the bank fees.
Let me go back and get this straight..... She has the money to go on vacation but not to pay her bills????? Makes me wonder just how good of care your horse is getting.