That is heart breaking. Surprise to him/her move so well despite its feet. Bless your heart for wanting to try to do something with him/her. I would try calling the local SPCA or humain society. This poor animal might have food and water, but the shape of it's feet I would think should be enough to get the horse in a better home.
If those people are THAT stupid, I would take it in my own hands. Go over to their place with a list of farriers and explain what the outcome of this lack of hoof care really means, make sure to let them know that if you see the ponies feet havent been done in the next 2-3 weeks, you will be calling local authorities for animal abuse(even tho realistically nothing can really be done, use the scare tactic). Explain to them you are willing to take the pony in for free so it can actually get cared for because they are obviously to green to care for it and see what they say.
Essentially it comes down to giving them 2 options. I don't what else to tell you because I dont know those neighbors. I know anyone like the ones we used to have would not give you the option of giving them a threat. Our old neighbors were psychos and it would have meant we were putting ourselves at risk of finding stuff on fire, or stolen etc etc.
Again, good for you for trying so hard to try to make a difference. I wish more people like you were around.