I appreciate you doing this quietly and without telling the quilter. There are simply so many reasons why a quilt top can turn out less than one would wish and your willingness to help out makes a difference in the end. Some of the members in my guild are quite vocal about people who "can't seem to sew a quarter inch" or "sew everything wonky". They have stopped participating in group projects that involve sharing blocks because they say that it's too hard to work with blocks that aren't made right. In my opinion, we all have our own level of acceptable and their words are not just hurtful to the quilter but to the guild as a whole.
I actually belong to two guilds, and in the other one the problem of members making wonky blocks to go into a group quilt was approached very differently. Rather than calling people out, a couple of members took on the project of doing a lesson once a month that was always followed up with something for the group to make based on the skill in the lesson. We have been collaborating on some great quilts and it is the more skilled/experienced quilters who take on the final assembly and finishing.
We have all been beginners. We all have life experiences that can disrupt our flow. Your kindness in recognizing this and taking care of people is wonderful. It might be worth exploring the idea of teaching some mini-lessons at your guild, focusing on the most common problems.