That bubble looks awful, it looks like the skin is pulling away from the muscle tissue underneath, it could be a giant blister from a possible burn, it could break open soon, or even get infected, the snake needs to go to the vet immediately.

It definitely looks serious, it could be something which could threaten the snakes life, please make sure it gets to a vet!

Try messaging Dr. Del, he is a vet who posts on this forum quite often...