I highly recommend suggesting that your veterinarian look up a few reptile medicine sources.
In the book, Understanding Reptile Parasites, by Dr. Klingenberg, he states that ivermectin is fatal in turtles because it crosses the blood brain barrier.
In the Biomedical and Surgical Aspects of Captive Reptile Husbandry, volume 1, by Dr. Frye, the only mention of ivermectin includes a statement in caps, DO NOT USE IN CHELONIANS.
Do not beat yourself up over this, if you second-guessed everything that the veterinarian did, you would quickly get booted out of the door. There are many sites on the Internet that have Reptile friendly veterinarians listed. Try to find one on that list and go there instead. In order to keep this person from doing any more damage, I would highly recommend printing out the sources I listed and giving them to the vet.
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*Humans aren't the only species on earth... we just act like it.
".the oldest task in human history: to live on a piece of land without
spoiling it."
Aldo Leopold (1938)