To the best of my knowledge, it is not something that can be undone -- if it truly is a fallen crest.
It may be improved with some of these things, depending on the animals needs -- weight loss, thinning the mane if heavy, pulling the mane to the opposite side and braiding/weighing it (or both), possibly some mineral supplements such as magnesium (has been reported to help cresty stallions, not fallen but cresty).
Why don't you post a picture both sides, so we can see what you are terming "fallen crest". That is actually when the ligaments have given way and you can rarely correct this entirely. It is, by the way, a hereditary thing. Have you had the mare tested for thyroid? Are you talking heavy crest, not fallen? That can be tremendously helped, even eliminated in some instances.
It would be FANTASTIC if someone has a method to truly correct a fallen crest. Well, I'm not talking surgical.