Lots of people do do the liver/gallbladder flush while pregnant.
However they are usually seasoned flushers and have also adopted a healthy lifestyle. Just doing a gallbladder flush out of the blue, without properlly preparing the body can cause some unpleasant symptoms. And, being pregnant, those symptoms may be quite exagerated.
What is her lifestyle like? What does she normally eat each day?
Is she on any prescription meds?