“The deed reads, boundary from main road to center of old woods road.”
I’ll make an assumption from that and your other postings that the woods road centerline is the boundary between you and the bank owned property.
So, you own the half of the road, and the “friend” has no easement on your half that you know about.
As others have said, you have no standing to challenge his use of the half owned by the bank, but he can be charged with trespass if he uses your half after being notified not to do so.
I would have a lawyer write him a letter stating that he has no permission or legal authority to trespass on your property. You don’t have to mention that the road is half off your property.
I would have a surveyor survey the line that runs in the center of the road, and get the bank to sign a deed agreeing the survey line is the boundary, not “the center of the road”.
Once you have such a deed, move the road over, or widen it onto your side of the line. That way you will have full authority to keep him off.
Make sure you put no trespassing signs on your side of the road on your property, and place an inconspicuous game camera to capture anyone destroying them.
If his horse poops on your side of the current centerline, take photos and document the dates.
Lawyers are not cheap, nor are court actions, so decide how bothersome is his use and horse poop.
The more I think about it, the whole horse poop thing seems trivial. While it is important to assert your property rights, and guarantee he doesn’t establish a right, perhaps you could grant him permission that you could rescind if he continues to leave a mess. I’ve run over horse poop and it has never damaged my vehicle or even smelled bad. Is that really the issue, or do you just need to assert your rights? By all means do so, it can prevent further, perhaps more damaging abuse in the future. Talk to a lawyer.