Depends on which is more important- the quilt coming out the way you think it should, or your relationship with them and their feelings.
If it was my sister, I'd tell her, "This is incredible, I can't believe you're already this good! With this batting, I'd quilt closer together though, because it has the tendency not to stay together... double check the bag, too, because different battings have different amount of inches you can have your quilting spaces. Sometimes, they'll get all warped in the wash if you don't do it close enough together... and I'm not criticizing, I LOVE what you did!"
My sister and I are close, and I'd never want to hurt her. I don't care if people are thick skinned or thin skinned, or whether they SHOULD be or not, I tippy toe because I hate hurting people in my life. Also, I don't think there's a wrong way to make a quilt as long as the creator is happy with the outcome. If they don't want it to last forever, I'm not gonna tell them they're wrong.