If you post on your local equestrian facebook pages there will be lots of suggestions for retirement livery in most areas.
Our pony has been at one for the past 6m due to a minor soft tissue injury that wasn’t quite 100% trotting on a hard circle, about 1/10 lame, but sound on the straight or soft. He’s had 6m off to see if it comes right with time. He's being reassessed in the next few weeks, to see if it’s better. If he’s still not quite right then he’ll carry on where he is, chilling with his mates. He is the most awesome pony to ever walk the earth (in my humble opinion!) and if he needs to be a field ornament then I’m happy to let him enjoy it. He doesn’t owe us anything. He is in his mid/late teens so could have another 5/10 years hopefully.
In our set up we pay £250pcm retirement livery. He’s in a small herd of similar aged/sized boys. They have access to field shelters, but live out 24/7. They are checked twice per day by the yard owner and the field is poo picked daily. We pay as and when for worm counts/ worming /feet trimming/ hay in the winter. That seems to be the going rate around here. There are some places that stable and turn out on full retirement livery but obviously these cost a lot more. I go and see him twice a week for a pat and a polo but all of his care is taken care of day to day.
I hope you find something suitable for your mare.
I do understand it’s really tough, especially with a teen (watching their friends out and about having fun on their ponies and feeling left out). I think people are very valid in asking what your daughter’s plans are re uni, as if she’s heading towards A levels and then leaving home, she’s probably only got another year left of riding (if she’s in y12), before having to concentrate on school work.
We had a situation several years ago with a loan pony we had for DC. He had an issue (unknown to us but pre-existing when he arrived) we which treated (at our expense) but sadly was still not quite right and no further treatment options. The vet hospital said he needed to retire or PTS. My DC was very upset but we sent him back to his owner as we had only had him a few months and I wasn’t prepared to pay for a long retirement (he was still young) for a horse that wasn’t ours when it was a pre existing issue. Obviously in your case it’s your horse so the buck stops with you unfortunately.
I also wouldn’t be going against the vets advice and riding her. If she trips and comes down on your daughter whether it’s in the school or even on a gentle walk hack then your daughter could suffer life changing injuries or even be killed. A friend of mine is still suffering from an injury where her horse tripped in the school, she came off and seriously damaged her arm, requiring surgery (I think the horse went down on her knees, but she certainly didn’t fall on my friend). The horse was sent for investigations, and there was an issue with her neck. Like yours, she wasn’t actually lame IIRC. After investigations and attempted treatment, she’s now retired on vet’s advice. It has really shaken up my friend (in her 40s, ridden and owned since a small child), and she has lost confidence in riding other horses because of it.