I think the people in this thread have had excellent advice. I personally would tell her when it is going to happen, let her say goodbye right near when it happens, but I would not let her be there when the horse is actually put down. It might be too much for her to emotionally handle. I always want to be with my animals at the end, but twice things happened right at the end that haunted me and I had a hard time coping with, so now what I do is have them put my animal into a sound sleep, say my goodbyes as they fall asleep, and leave for the final injection. That way I'm there for them to the end of their consciousness, but don't have to watch them stop breathing. I've watched many, many, many animals pass away (being a wildlife rehabber, this is a common thing for me to witness), but it's different when it's your family member.
Obviously putting the horse into a sleep and then having her leave is probably not an option in this case, but the point is--I think it would be better if she has a day or two at least in advance to say goodbye, and then let her stay with the horse until it's time for the actual injection is administered. That way she gets to stay with the horse until almost the end and say goodbye, but doesn't have to have those horrible memories of watching her beloved friend actually dying. With horses I just worry it could be very unpleasant and sad to watch.
When we put down my horse Salty at the age of thirty-three (I was twelve or thirteen at the time), I went and said my goodbyes, and they put him down about a half-hour later. This worked fairly well for me.
I hope whatever you decide, it goes smoothly and your horse is able to pass in peace. It's always so sad and difficult making this decision, but in the end of course it's much better than allowing them to suffer.
Obviously putting the horse into a sleep and then having her leave is probably not an option in this case, but the point is--I think it would be better if she has a day or two at least in advance to say goodbye, and then let her stay with the horse until it's time for the actual injection is administered. That way she gets to stay with the horse until almost the end and say goodbye, but doesn't have to have those horrible memories of watching her beloved friend actually dying. With horses I just worry it could be very unpleasant and sad to watch.
When we put down my horse Salty at the age of thirty-three (I was twelve or thirteen at the time), I went and said my goodbyes, and they put him down about a half-hour later. This worked fairly well for me.

