I can't agree with this. If this wasn't a mom would you say the same things? Sure maybe not the 'tenth degree' but they need to be criticized not enabled.
This animal is being neglected. Even after being given advice from both the boarding location and child, they shrug it off with the 'I know best' attitude and rude comments. That's not a doing the best with what they had.
55fingers:
Issues with family are always difficult. I'm sure that you already know this and I hope that things are better now that you're living with your father. I hope that in time things improve for you all, but I know that it isn't always possible.
The sad thing is, there really isn't anything you can do outside of offering to buy Dora. Even then, your mother may use that in the future for gaslighting or guilt again. You left to live with your father and the animal is now back in her care. Dora belongs to her.
Good luck regardless.
At the very least, consider sneaking in some vitamin dusted bugs or good veggies when you visit. Something to be an improvement to what she's having. At 8 inches at this age, she's most likely permanently stunted from malnourishment. But she can at least have a bit of nicer QOL when you visit.