Until you get to the end and find there's one extra 'M', and one missing 'R'. So need to go back through all of them to find where the mistake is.
I was going to make an "all those extra pieces" remark, but I thought it'd be in poor taste.
Simple solution: Print a scale 1x1 keyboard layout on paper (spanning multiple pages is OK) and then extras on the side.
Do one set at a time, then bag them/box them as you ensure each set is good.
Anyhow, as long as he uses some organized system (maybe marking the bags ONE or TWO - some keys come in pairs) and takes things slow but steady, he should be fine.
Again, there's no reason for Signature to be ****s. They didn't collate (extra $$$), and the only logical way to manufacture is to do one keycap (e.x. A, CTRL, ALT, etc.) at a time. Bag 'em by key.
50 sets? Quick to filter if you're reasonably careful.