I've had a couple customers bring that batting for their quilts- it is very lightweight so the heavy question- no, the batting won't make the quilt heavy.
you will need to really, really baste well though. Quilt from center out on a well basted quilt, if pin basting I would say every 3-4", if thread basting I would probably baste a 2" grid. I wouldn't recommend spray basting for this.
She might not have any idea what she was buying. Call & talk to her about it. It will not make the quilt stiff at all quilting the recommended amount. I found it not that bad to work with.