In the phone call, it is understandable that you couldn't say anything the daughter would hear, and educating the mother would be a big job.

But it's also important to remember that kids aren't stupid, they are wired to learn, and sometimes all it takes for a kid to put something in context is to get that an outsider thought it was unreasonable too. I have found, in malls, for example, that simple eye contact can make a big difference. The kid gets the message "yes, your parent is being unreasonable. Other people see it that way also." It's not as rewarding as fixing the parent in real-time, but it can be a real help, not to be dismissed either.