How many of us have been there, right? You get in a hurry and think the glue is set, but it's still a little soft underneath. Then you sand and it just gums everything up, leaving that rough, gritty texture that ruins the whole piece. I did that on a picture frame once and had to start the edge all over. It's such a frustrating waste of time. Letting it cure all the way makes such a huge difference, so your friend's mistake is a good lesson for everyone. Patience really is the key.