I'm trying to get an ex racehorse used to trail too. I think the feeling of being out in the open frightens her (she drops a shoulder and spins when reacting to stuff). She was boarded in a commercial riding school barn and from being at the track is used to noise. But the trail flips her out. (Bunnies, like someone said! Yesterday it was a hawk.) I introduce trail work slowly with a horse like these (as we really don't have many quiet trail horses at our barn. One sets off the other and it's a domino affect out on the trail!) I like going out alone to start, horse in hand, and AFTER a long ring session. So she's tired already. Or I start trail work in the summer when it's hot then by the time winter arrives, she should be pretty comfortable. Then again, I think that there are some horses that never really have the mentality to become trail horses. We have a 24 year old TB at our barn who just isn't good on the trail even though he's only serviceably sound. That being said, I think you can plug along and keep reintroducing it and sooner or later she may become a good trail horse. The ex racer I'm working on trails used to bolt, lunge spin and bolt and hop in this really weird semi rear. Now she shies in place and I don't punish it, I just press her forward. But like someone else said, you're working against the horse's training with a fearful rider. That's going to hinder your progress as the horse looks to the rider for confidence. Maybe you can work out a relationship with the other rider where she agrees not to ride him on trail while you're working with the horse.