Well, it could be that the Tbred is suffering from an illness, worms, ageing or something else other than just lack of "general" care and the owners are unaware of what they could do to help him. If you are uncomfortable approaching the owners and you really think that the horse needs help, your only option is to call the SPCA and ask them to stop by again. But, I would, when talking to them, tread lightly -- suggest to them that you think the owners need some guidance, not that they are totally ignorant or (heaven-forbid) just don't care.
What seems funny is that the pony is overall OK, but the Tbred isn't? Maybe they are trying to rehabilitate the Tbred?
Personally, if I'm not willing to walk to the front door and offer my help, I don't ask someone else to do it for me. However, in some situs that could be dangerous so be careful, whatever you do. Much of a solution comes from the way the problem is discussed.