If it is high ringbone, a magnetic bellboot (at least the ones I'm familiar with from like 10+ yrs ago) won't really address that issue ANYhow. They used to make magnetic pastern wraps or whole-ankle-and-foot boots, dunno if they still do but that'd be more geometrically appropriate for your problem if you really want to try magnets. (wait, I just googled, various companies do still sell magnetic pastern wraps or cuffs).
Personally I think the chance of the magnets making a difference in this type case is very very low, but, doesn't hurt to try it I suppose if you can afford.
Your best bet is probably to continue pursuing as excellently-correct trimming (and shoeing, if she's shod) as you can, since IME that tends to make the biggest difference.
Good luck,
Pat