I would never put a horse down because they can't be ridden. I figure if, say, he is 25 when he can no longer be ridden (although I know they go a lot further than that) then he darn well deserves to live his last years doing whatever the heck he pleases in my pasture. He gave me at least 20 years of good hard work, so I will give him his last few to enjoy himself, and run as he pleases without being ridden.
Of course, if his pain cannot be managed you put them down, but arthritis is not unmanageable in most cases.
Honestly I think too many people throw animals away or put them down because they do not fit their "purposes". If you can afford to get another one, you can afford to retire your old one.
I'm not talking about competing, or even daily rides. I'm talking about capable of being tacked up and taken for a trail ride or something. Horses are happiest with something to look forewards to, and to keep their minds sharp. That's how our two mares Delriah (49) and Puddin' (going on 28) have made it this far. The exercise keeps them in great shape.
So yes. Really. I didn't say that I would put the animal down as soon as it developed artheritis, I said that I would put it down when it was no longer capable of being ridden without pain. Frequent trimming, a good diet, and exercise can all keep a horse with even moderate to severe artheritis healthy and able to go for a trail ride or do some arena work with no discomfort. Once it progresses to where the animal is tripping on it's own feet (not a sign of artheritis, but of nerve deterioration) and unable to feel where it's going, then it needs to be put down.
Artheritis can be doctored, nerves can't. It's sometimes painful for a horse to begin loosing feeling, and it can be very dangerous and scary. At that point there is no reason not to put it down.
OP- this is exactly what I'm talking about. Perhaps she is being 'unjust' or 'cruel' to her horse, but its not because she's trying to sell him. If he's nearly faceplanting every time he's ridden, he's dangerous to himself and his riders. In a pasture he could hurt himself, and in an arena he could hurt himself. This is one of those circumstances where I would think that putting him down is what is best.