Just kill them?
I hope not. I am more crippled than either one. I live in constant pain -- if I think about it. I am pretty good at 'tuning it out'. I take very few pain meds and use a lot of Flexiril (muscle relaxers) which helps with muscle spasms from my Scoliosis and my lower spine degenerative joint disease.
There is no treatment that will make either of these sweet old mares less lame. Both are happy and fat and live a happy herd life.
I have had many crippled horses over the years. It has been my finding (and that of my Vet I used for 25 years), that if a horse is eating well, not laying around a lot, holding weight well and keeping up with the herd on pasture, THAT HORSE IS NOT SUFFERING. I think most lame horses tune out all but the most severe pain. When they hit a point where it starts to bother them, they do downhill fast and it is time to put them down. That is exactly how we handle it here. As long as I can afford to feed and keep the pensioners, I will.
What I find very unsettling is when people force horses to work when lame or give high levels of Bute or its equivalent to a horse and work them anyway. I would be much more upset with the OPs horse owner if they were Buting it and using it in lessons anyway. Then, I would have a problem with it.
The reasons I would not report 'mildly lame' horses and make a huge deal out of them are:
1) I think animal rights groups pounce on every opportunity to make livestock and pet owners look bad. I think they use every opportunity to advance their agenda that calls for an end to all animal ownership and use. While they are not real big right here, they are in a lot of places.
2) If you start calling authorities for every little thing, the chances are real good that they will ignore you completely when you really need them. If you think this horse is neglected and looks bad, you need a reality check by going to local horse auction. You will see about 75 horses that look really pathetic for every 15 or 20 you see that even resemble this one if you would go to the local sale right here in town and they will not even let people unload very thin horses or crippled ones.