Misoprostol was prescribed to remove few products of conception that was still left over in uterus.
Urine
pregnancy test detects Pregnancy hormone called "
Beta HCG" hormone that is excreted by kidneys into urine.
It will take few days to weeks for her urine pregnancy test to become negative.
Next Steps
Just get another transvaginal ultrasound to check if the retained products got expelled or not.