Differences in rating 'systems'
My friend and I recently begun using duolingo. He already speaks high level french so it is obvious he advances through each lesson faster.
What I'm curious about is that, even when he makes mistakes, if he reviews his lessons, instead of having 'hearts' left on each book, whether 0 or 1-4, like I do, he has the squares-bar that is identical to word strength but never the hearts.
Likewise, on the learning tree 'overview' page for the french language, his completed lessons do not have progress bars like mine where a black 'filling liquid' fills up the horizontal tubes. He only has those horizontal rows of square blocks that fill up in gold colour as he moves along.
Last but not least, how do you achieve a perfect rating in any given lesson? I've seen my friend achieve perfect rating in many lessons although he has committed mistakes throughout them.
In the beginning you have 4 hearts, in later lessons you only have 3 to lose. It also sounds like your friend might be one of the users of the new system that Duolingo is testing right now, and you are not.