Unfortunately no, the mare doesn't receive anything for pain relief.
They decided to euthanize just the horses in a very critical condition, the ones that can't stand on their feet and the ones that are in very very bad condition. They decided that this mare can live her life in a decent way. Anyway, I have some cases more critical that this one. This why I hate my country, for it's lack of laws for animal protection . If even in a shelter is happening this, you can imagine what it can happen in other places.