Jesus Christ, what a freak accident. So sorry about what happened, how large was the sponge, also keep in mind most of a sponges volume is air and water, so it should shrink significantly during digestion, it's possible that your snake will vomit it up in chunks. It might be a better idea to try and induce vomiting, I don't know how you could do that safely with a snake but it might be a good idea because it could have a lower chance of being impacted. I'm not a herp vet, but that's the advice I can give you. Hopefully he/she comes out of it ok.