Check with the other rider (gently) how she normally behaves, does she get strong? Do they just let her go as fast as she likes? Do they have certain places they always gallop? Etc. You may find they have inadvertently trained her to do this. There’s a big difference between being carted off with and a truly bolting horse.
Reschooling wise I’d just do 100’s of transitions every ride, walk to halt until she’s really soft and only then introduce faster paces. I’d probably keep it in an arena or small paddock initially too so she can’t get a head of steam built up.