You need to be careful "pushing" it back in. If you fold it onto itself, you will kill that part that is overlapped, thus killing the snake. If this has happened once, it will probably happen again. Like someone said, this one may need a stitch to keep it from prolapsing again.
I can't tell you how bad of a prolapse this is. I would like to see more pictures of the prolapse as well as the vent. I'm also concerned that the owner didn't know what to do immediately and I'm not sure there hasn't been some damage already. I can only hope that he got on BP.net immediately and got answers really quickly.
I can't believe a vet would send that snake home with nothing more than antibiotics.
Jim Smith