Hindsight is a bitch. In short, she had no idea what kind of rake she was stepping on, and so saw no need to dissemble. She matter-of-factly told them they were welcome to take what remained, which was about 1/5 of what had been there — and then she got the letter from their lawyers, by which time it was too late.
True, but the thing with verbal agreements is that it's very difficult to prove what was agreed upon.
Your mother should have just lied.