I think the dress isn’t sized correctly for her. The top fits, but the bodice is too tight. A dress like that is not really meant to show the dent where your bellybutton is. I think this detracts from the line of the dress. Spanx could be one solution. Going up a size and having the top altered to fit would be another.
But if this girl doesn’t mind the bellybutton divot and thinks that looks okay, and most importantly, loves how she feels in the dress, then nobody should push anything else on her. I think this is where the Dillard’s saleswoman fucked up. It’s not Spanx she should be trying to upsell, it’s Dillard’s alteration services. “The line of this dress is designed to not quite fit this tightly through the middle. If we go up a size, the straps and cups might not fit, but we can alter that right here and it only takes three days! Would you like to try a different size to see how it looks on you?”
This blames any problems someone might perceive on the dress, not on a girl’s body.