By no stretch of the imagination is the woman fat, so it isn't fat-shaming.
The fault lies with the clothing industry as.a whole. We need standardised sizing preferably expressed as measurements, e.g. waist 71cm/28 inches.
You can't rely on 'size 12' to mean anything, much less 'medium' or 'large'.
In the pictures, it does look rather snug on her so she should take some responsibility for not sending it back and replacing with a size up.
But it shouldn't fall apart to that extent even if it's too small, so I still think it's mainly the manufacturer at fault.