It's always upsetting when you find a mistake in a quilt you made. Step away for it for a while & get your emotions out of the way. If it is for a wall hanging, you may want to take the time to rip out the quilting stitches, undo the error and fix it. But a lot of non-quilters don't recognize block layouts,so she might not realize there is an error. If you leave it in, it will bug you everytime you see it. If it is to snuggle with, it won't be noticed.
I have also discovered that taking a photo of quilts makes the mistakes jump out at me so I can fix them sooner.