2 different sized feet

So i have been working at my local ski shop for some time now, and today while doing a rental i came across something I have never seen before. the boy was 8 years old and was in an accident that caused him to lose some of his foot, I didn't get to far into it with his father he just summed it up and we were saying we were glad everyone is okay. Anyway when i was measuring his feet they were two different sizes due to this, a 21, and a 18
He has a custom made footbed in his daily shoes that he showed me, it basically supports him, and at the toe of it is basically toes so he fit to the shoes he was wearing aka, you can't go around buying 2 different sizes all the time so it "made" his bad foot, equal length to his bad foot. Not that it is a bad foot just how i worded it. Anyway, I was wondering if anyone has come across something like this before, and how it would be handled if you were boot fitting. The kid and I ended up having a blast, I gave him and his brother posters, and he was so hype and didn't think about his foot twice, just wanted to ski with his family. Love and respect to them. Just looking to expand more boot fitting knowledge and what some of the pros on here would do if they came across this situation or have had one with an adult.