OK, I was asking because this tree looks a if there are more problems than just overwatering it one time. If it where to be overwatered once you would see the leafs turn yellow before they fall off. This looks as if the roots suddenly started to rot and the tree isn't able to feed itself anymore. This could either have been caused by a recent repot or by a slow decay of the root system and triggered by overwatering. As you say this tree hasn't been repotted lately the tree must have been very weak before you overwatered it, because a healthy azalea must be able to survive a short period in wet soil. So in order answer your question if this tree could be saved, we have to find what caused this tree to be so weak in the first place.
With this being said, I think that your soil mixture of peat and coarse sand is far from ideal for growing azalea's or growing any tree inside a pot for that matter because it just holds way too much water. Azalea's should be potted into 100% kanuma soil. In structure it looks and feels the same as akadama. It holds about the same amount of water and air too, but it's much more acidic as azalea's prefer this. So this is one reason I could think of that might have weaken you tree as it's very easy to overwater you tree in this soil and you might have done it numerous times before without you noticing it. It might also have suffocated due to lack of oxygen in the soil. If this is the case, than I fear the worse, as I don't think this tree will survive another repot within six months after the previous in this state.
But there might also be other causes. Could you give me a detailed description of how you have taken care of this tree in the last six months? Think of its position. Was it inside, of outside? Did it get a lot of sunlight? How often did you water it? Did you gave him a lot of fertiliser? etc..