R.D.
My son should have been in a helmet; I wish the pediatrician had pushed it more. He's 22 months old and his head has rounded out as much as it will... he'll never want to shave his head like daddy ;) It's not like you'd notice it unless you were looking for it, but the flat spot is so very definitely there.
Just wanted to share that, and encourage the helmet if it's recommended!!
ETA: If she's dead set against it, I highly advise her to weigh the pros and cons. Want me to send her a picture of my son's dome piece!? It won't correct itself, and her poor daughter will have to live with that forever. She'll regret not doing it; I do!!