Supplements only mask the symptoms of the problem--and IMHO are a waste of money--and you actually want to solve the problem, completely. To do that, your daughter and her mare should find a competent natural horsemanship trainer to work with both of them, together. The trainer will show your daughter how to desensitize her mare, and more importantly, how to improve the relationship between the two of them.
The connection between horse and human involves the human being the leading member of the partnership, and as the leader she makes the decisions. That means, in part, that the human figures out what it safe and what is harmful, and will be the horse's protector. The mare is spooking b/c she feels as though she has to be her OWN protector: unless she knows for certain that something is not harmful she does what any prey animal would do, she gets the heck out of there. She spooks at everything.
But if the mare knows that your daughter is watching out for her, and would never ask her to do anything or go anywhere that's dangerous, she won't spook. You can't desensitize a horse to enough things to make sure she never spooks at anything, but you CAN teach a horse that she doesn't need to spook b/c her handler is her protector. Most horses want someone else to make all of the decisions--they need a leader--and they live in herds so that somebody is always watching for danger. In their small herd of two, your daughter watches for danger, but the mare needs to learn that she can rely on your daughter for that protection.
I only suggest a NH trainer for your daughter and her mare b/c NH trainers know so much about the horse/human connection, but some other trainers know, too. If a trainer says you can drop the mare off and she will be fine when he's done with her, walk away and find someone else. Both your daughter and her mare need to work together on this.
Until you find a trainer, you can work on desensitizing the mare by taking her to schooling shows, to other barns, even on trail rides. Expose her to as many unusual things that you can. Improving the relationship between your daughter and her horse takes too much to be covered by a forum, but books and DVDs by Monty Roberts, John Lyons, Mark Rashid and Stacy Westfall could at least show your daughter what kind of connection she COULD have with the mare.
Please don't rely upon supplements to mask the problem. As you've seen, they don't solve the problem. Once a horse and rider become partners they can do anything together. It's an amazing journey, and it starts with something as easy as groundwork. I'm sure you can get some tips on doing groundwork from books and DVDs, but it will be easier and safer to work with a trainer. Good luck.
ETA: Does the mare live alone? Horses are herd animals, and even if your daughter becomes the mare's protector, your daughter cannot be with the mare 24/7. If the mare lives alone, she should have a companion so that she can rest sometimes from being on guard. That may be why she's pacing: she's looking for danger. You don't need another horse, although that is preferable. But a donkey, a miniature horse, even a goat can be companions.
