Friend: “Dobbin was bucking on the trails again today. I am not enjoying riding him much”
You: “Dobbin might benefit from being able to exercise himself in a paddock or field on the days you cannot ride. He is likely bored being stuck in a small space”
Friend: “Dobbin was spooking and reacting to every little thing”
You: “Dobbin might benefit from being turned out with friends so he learns to relax - there is a nice barn near you that might really help”
Friend: “…”
You: “I really value your friendship, but the issues you are having with Dobbin make me worry about your safety. I very much think a new board situation would be helpful for you so you can start to enjoy your horse”.
After that, you can answer any further conversations of the like with: “We have already discussed the issues you are having with your horse before. You know what I think would help.” and then change the subject.