If I am interpreting the pictures correctly, he made fatter grey pleats vs more narrow ones in the center sections? I would run by him that in the picture they had more narrow pleats and is there a way he could fix the covers he made to reflect that? I would not mention cost or paying for it, and just see how he responds. Let him decide the direction of the conversation and don't worry about conflict as if he truely missed this, he will want to make it correct. He might ask more money for the extra time it would take, or he might do it gratis as it was his mistake. One way or another he will be more carefull in the future, you won't harber any regrets, your friendship should not be questioned as you are not complaining or cutting down his work, just pointing out how the picture is what you wanted to start with, nothing more nothing less, and how you really liked the quality of his work and value his friendship. Honesty goes a long way to forming even better friendships.