I agree with sending a letter to the last known address, if she's say in a hospital or something, then someone else should be in charge of dealing with things for her, and should be able to either get the letter to her, or take care of it themselves. I like to give people the benefit of the doubt, but at the same time, she should have contacted you herself, or had someone else in charge contact you, and make arrangements for the horse. After sending the certified letter, I would wait a week or so, see if you get any response, and then hopefully others will have an idea of what you can do legally with the horse if no contact can be made. Best of luck to you. Its hard getting after a friend, but if you can't afford to take care of two horses, one that's not even yours, then you need to do what you need to do to get that horse off your plate financially.