I have to fully agree with Dr. Del! I know I'm a new user to this site, but I have been keeping and breeding reptiles for over 20 years and have come across this before with rescues. The bulge appears to be past the stomach which would indicate intestinal compaction/twisting, possibly from the previous owners choice of substrate and lack of water combined. The swelling is most likely methane from the decomposing rodent that is stuck in the intestine. If it is compaction, the only home remedy I know of is super soaking (water logging) and massaging the snake. This has helped me in the past, and it has failed as well. The best chance the snake has is to see a qualified vet.
Good luck and keep us informed.