I have a "quick and dirty" solution to your scale problem. Get a second scale, have her stand with one foot on each scale, then simply add the two weights together. It won't be 100% accurate, but it will be close, and a lot cheaper than buying a specialty scale that weighs over 330 pounds.

An added advantage is that this will give her a terrific target to shoot for...the day when she can use a single scale to weigh herself!

It sounds like you are both making great progress...keep up the good work!