Hello and welcome. My name is ***** ***** I'm a biologist with a special interest in reptiles. I'm sorry to hear of your turtle
If the vet you saw was an actual reptile vet, you can probably go with this advice. If you saw a local vet who treats mostly dogs and cats, that is a different story.
When a shell cracks, that opens the body to infection. This is especially true with water turtles because there are always bacteria in the water. Normally, a vet will treat the interior to destroy bacteria. Then the shell is repaired. The turtle is often placed on antibiotics. Here is a site where you can read about the complex procedure in detail:
A cracked shell can be life-threatening, so if you didn't see a qualified reptile vet, I recommend that you do that. These sites have directories of vets who treat turtles:
If you have more questions, let me know in a REPLY. I hope your turtle will heal uneventfully.
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