boy, that is too bad :(
i keep a mirror tile right on my quilting table and as soon as i start i use it to look at my stitching on the back and i check it again each time i change bobbins, notice a new noise, stop for a few minutes...i am always taking a moment to walk around the table and check to see how it is looking...because
i did that once, only about 2 feet wide before i realized it was not stitching well, but enough that it took me almost 8 hours to pick it all out and then had to re do it...now i check so there is never more than i strip across the quilt and usually less because when i turn the quilt and start again i go 4-6 inches, stop and check it...if all is well i continue...it is a hard lesson to learn.
i give classes on my machine and rent out time...i always stress taking this step...sometimes people don't take me seriously...and i will stop by the machine to see how they are doing and i always pick up the mirror and look at the back; sometimes i have to tell them...oops better stop what you are doing, you have a problem...see this is exactly why i told you to take a moment and check.
sorry you have had this experience...i would offer to help you fix it if i were the friend who did this...have you talked to her about it?