Um how would night weaning affect weight? Id brush off everything she's told you and keep doing what your doing if it works for you. My Ds was a night breastfeeder til he weaned at 13mths and still wakes during the night for milk................and he is 2.5 and a hefty 16.5kgs; even at 12mths and night breastfed he was a good 11kg.

If she is that concerned about your daughters weight growth she should tell you to see a paeditrician to investigate other issues.