I personally find it sad that the mother won't go the extra mile for a horse that has served her daughter so well for years and years. People should be more considerate about senior horses.
While it is easy to say this, the reality is that running any kind of horse business and deriving your living from it is very difficult at best. No one 'wants' to put down horses that are no longer 'useful'. But, every single horse business has to support its owners, employees and 'needed' horses FIRST. It is just the reality of being able to stay in business and to put food on YOUR table and feed YOUR family.
I have always derived my living from horses. It is all I have ever done and all I know. It has become increasingly more difficult as my severe arthritis has limited my riding ability. My riding and training ability is what has kept food on my table (and several other persons' tables) for over 50 years.
I currently have 4 horses on 'pension' -- totally useless but served me well for many years. Three I raised and one I've had since she was 3. Thankfully, I am still able to do this. It is getting more and more difficult for me. If I get to the point where it is no longer possible, they have to go, probably along with 3 or 4 more that really do not 'pay for their keep' any more.
If it come to that point, it is a choice between keeping the quality of life high for 3 or 4 useless horses and being able to keep and feed the 40 others that I have.
These older horses do not include the 10 or 12 mares that I have kept since 2008 without breeding until last year. These are mares that I refused to send to the sales because I knew the only bidders on these good, big, fat, pretty registered mares would send them to Mexico. Hopefully, I can sell some of them in the near future to people that want to produce that kind of saddle horses.
So, I do not condemn anyone that does not want to keep and feed and care for a useless horse. I think they are livestock -- no matter how much we like some individual horses. I think no one should have their property rights taken away from them or told they MUST feed and care for a useless horse nor thought less of because they do not want to keep one.
In this case, it is now up to the daughter -- the horse's actual owner. She should bear the cost of proper upkeep or have the horse put down. The mother has done everything she can and should do under the circumstances.
And I agree that it is not 'worth' an expensive Vet bill, which would have to include a very expensive evaluation and lab work-up. In this case, I cannot think of a single reason doing this for this horse would 'fix' anything that can be corrected by Veterinary intervention at this point. Finding the cause is only an expensive way to satisfy curiosity. There is not one single thing that would add to this horse's health, comfort or usefulness coming from spending $1000.00 or maybe more and then adding a Vet fee to euthanize him when he got worse. I am sorry to say, the consequences of old age in an equine are not reversible. Everyone just has to make that decision for themselves at what point they have to stop putting money in a hopeless money pit that is miserable on top of being useless.