I think the part that worries me most is the fact that there's other animals involved. I hope that they at least come back relatively 'okay'. I wouldn't wish it on anything but I hope it's just a matter of neglect as opposed to neglect and infectious diseases on top of that.
I also have my fingers crossed for you that this guy will start getting some body weight back and that he comes back with some decent labs when he's well enough for them. Hopefully there's no long term damage to the liver or kidneys from the malnourishment.
Regarding the vitamins, while I do agree that most ball pythons are relatively good when they're on the correct size of properly raised feeder rodent... I think doing some minor supplements in this case will definitely be a benefit to help jumpstart this guy. Glad the vet is on board with that as well. Eventually he can probably be weaned off those as his meal size increases.