Is there 1/4" beyond the points of the corner squares? If so, I would sew binding to the front side with a 1/4" seam, turn the binding to the back and hand sew the binding to the back. If you don't have 1/4", I don't think there's anything you can do.
Honestly, I would not want a longarmer doing any cutting of my quilt without my express request. She may not be aware that there are many different ways to bind a quilt. Personally, I like to "square" my quilts with marker lines and then sew my binding on *before* I trim the quilt sandwich. Works a lot better for me. My cookies would be frosted if I had to sew binding on 1/4" away from the edge of my quilt sandwich.
Your longarmer needs to create a one-page contract that specifies things such as whether you want the quilt squared up or not. It's not something that should ever be done to someone else's quilt automatically, but is a nice freebie for someone who would appreciate the service.