Sounds like a retained shed amputated the tip of the tail (imagine a rubberband around your fingertip). In most cases the dead tissue will die and fall off and the wound will heal up on its own. But you're welcome to put Neosporin on it if it makes you feel better.
You should always check each shed skin to make sure both eye caps and the tip of the tail are present. If left on it can cause problems, such as what you're describing.
Not much else you can do now, whats been done is done. Just make sure you try to rectify the husbandry and prevent future problems. Most retained shed problems can easily be fixed by having the snake slither through a wet paper towel or a wet wash cloth several times. Just make sure that you do it soon after you notice the problem, and not wait too long.