I disagree that perfection is the primary aim in quilting. Enjoyment is the primary aim in quilting (I'm assuming no-one here quilts because without it their family would not have bed clothes).
Sure, aim for perfection, there is nothing wrong with that, but not to the exclusion of everything else.
A good teacher should be able to tell the difference between someone who is doing their best, not getting perfection, but is OK with the learning experience, and someone who is not getting perfection and is therefore getting discouraged.
For a beginners class encouragement should be the order of the day - whether that is in relation to the 'can't see it from a galloping horse' category or the 'try again but do it this way' category.
It doesn't sound as if your SIL is getting the encouragement she needs, so I would vote for finding another class and/or send her here!!!