I think I read this somewhere on the forum before..and it was concluded that it was the skin on the avocado that is dangerous??
How did you feed yours Meg?
I tell people when I see this question come up, that if I had done this to a younger tortoise, I would not have been surprised if that baby had died. Nigel was around 10 at the time, and WC so his system was a bit more hardened, and apt to bounce back. I felt guilty about giving Nigel that "treat" for weeks, and it really did a number on his system. He had diarrhea for a few weeks afterwards regardless of going back to his strict diet.
Some tortoises seem to be able to stomach it, but after what it did to my Greek, I don't advise feeding it at all.