Dawn,
There could be a couple of situations.
First many times when new to longarming, and if she is being mentored by someone from the ‘old school’ she may be having the backing too taught. When I first learned how to use a longarm machine we actually could bounce quarters off the backing, that is no longer a recommended method.
Second, her needle might not be dull, but it might be larger than needed and if the backing’s thread count is high the needle has to go somewhere, so it will sometimes split the thread of the fabric and then with any push or pull it will finish shredding. Sometimes the shaft of the needle is larger than needed for the thread being used. I have also been noticing that MK from Florida has been promoting the use of ball point needles. I haven’t used them, but it’s a possibility,,,
And the other possibility would be a burr on the needle, not very likely, but if it’s very tiny it would be difficult to see.
I would also look over the backing where there are no stitches, run my hand back and forth a bit, see if for some reason, the fabric itself gives way to a stress other than needle moving through it.
None of these might be a solution, but each is worth checking out.
Tell her I wish her the very best in her side of the creative endeavor of quilting.