When I see "companion only" ads, I look really hard at the why? of it. Why are the unrideable? Why is the owner trying to turf her problem off on someone else? Why isn't she getting more/additional specialized care for the horse? WHY WHY WHY? And 99% of the time, when someone has a horse that's been unrideable and they want to sell it or give it away, it's because they have given up and don't have the spine to put the horse down.
I'm all for someone like Wallaby, who has made the commitment to work with her horse to try to get him sound of mind/body, trying everything in their power for a cherished animal. What I take issue with is when they have done what they can do and said, "That's it. I'm done.", whether for financial, emotional or just plain can't find another thing to do for the animal, they then want to have someone else do the dirty work. Let the horse go. Put him out of his misery and pain, regardless of whether that pain and misery is physical or emotional, let him rest.
I had a very difficult mare once, I bought her after waiting for her to be born for 3 years. Got her as an unhandled weanling and made excuses for her for 4 years. We didn't bond. I was too gruff, she needed a lighter hand, she was just 'high'. Sent her out to a trainer and got told she was terrorizing the other horses and the barn staff and the little kids would walk 20 acres around her stall to avoid passing near her, would I please come get her. Long story short, she injured herself with her terrorist tactics and I ended up at OK State University with her to find out what was wrong and how bad it was. Evidently, she had kicked at the horse next to her in the shedrow and managed to get a tiny little puncture wound that unfortunately went to the bone. She got an infection and MAYBE after $10K or so of surgery and hospitalization, would be broodmare sound afterwards. Like I'd even consider breeding that on! Anyhow, the point of all this is, when I was discussing her behaviors and the options for treatment the surgeon looked at me and said, "You realize that horses can have mental illness, just like people, right?". I put her down right then and there. I SHOULD have done it 3 years earlier, we'd have both had a lot less pain.
If this horse has been deemed unrideable, unsafe, then there's only one safe answer for him and it's not as a companion, IMO.