As far as I've read and been told by my pain management doctor, vicodin/lortab (hydrocodone) is generally safe in pregnancy. Although they really haven't done enough research into it, if it is neccessary in pregnancy, they generally consider it safer than some other meds.
I have to take stronger pain meds every day and am hoping for the best, myself. From what I've read, it's relatively safer than the stress and agony caused by constant pain.
I hope she gets some relief, too!
This is a question that you need to ask the doctor that did the root canal. I took viocin when I had a tooth pulled early in my second trimester and my baby was absolutely fine and the doctor that pulled my tooth knew I was pregnant and I even called to double check to make sure it was okay and she said it was fine. Hope your daughter gets some relief.