There is new research that shows that injection site is not as important as we used to believe, most young vets don’t even worry about the whole bottom third issue, which older vets always warn about.
Soaking is the fastest way to hydrate a dehydrated snake, snakes do not have a large subcutaneous layer like dogs and cats do so SQ fluids are not effective, and veins are very hard to hit let alone place a catheter in so I.V's are virtually impossible for a general vet to do on a snake that size. There are some zoo vets and exotic specialist that can do it however they are very expensive and hard to find.
Amikacin can cause renal failure but it is also a great drug that most reptile RI's are susceptible to, that dose however was correct and should not have caused any further problems unless the renal function was compromised in the first place.
I am very sorry for your loss however I don’t believe the vet is at fault in anyway. It just sucks that it was your friend’s snake that didn’t pull through with a decent treatment plan. If you would like to research anything I said feel free to look up the info on www.vin.com if you have vet or a tech that can get you access.