Wow, ChocNut, so sorry.
If you feel that being by her side is the most important thing, then stay with her. However, I'm not sure if this really matters that much to the horse.
I've held a horse being put down when I worked for the vet clinic. Its not hard, you just need to make sure to get out of the way.
If you are the type of person that will carry a scene around in your head, then maybe don't do it. It is memorable, and not in a good way, to watch a horse go down. It doesn't hurt them, but it is hard to see. If that will stick with you and cover up the good memories, then have someone else there.

Best wishes to you.



More clicks please!