Hi Emz77,
It certainly is a sad story and how devastating for your neighbour. :-( and the poor dogs.
I'm sure that the vet did do everything that was needed unfortunatley with poisons it is hard to treat not knowing what the poison is, making the dogs sick and the injection is the best course of action and then it is a case of waiting to see. The Yorkie being smaller would have been affected much worse with his little body. :-(
Going on from sara1bee's post I am sure that if the vet could have offered any other treatment he would have, money can be paid off in instalments if your neighbour did not have insurance, a vet would not put a life before money I am sure it would be against their moral ethics. With poisons you just have to hope that you can get it out of the system, hospitalization probably would not have helped some vets may have done it, (it is extra money as you say) but I doubt there was any other treatment to be given apart from being watchful and having your fingers crossed that the poison was vomitted up. When the vet assessed that the poison was only destroying the little ones body and continuing suffering then it was best to put the poor little one to sleep.
I would make sure that the poor little dogs story is highlighted in the neighbourhood to make everyone is fully aware of what has happened and hopefully the people responsible will not do it again, how can people be so silly and reckless.

It is an upsetting post, but you have highlighted the signs of possible poisonings for others to act on quickly. With poisons every minute counts.