She’ll claim it’s due to breastfeeding if pulled up. As lots of us have said before, although the milk gets the goodness first, if her diet is poor, the milk will be too.
I fed both my children, I ate right and although they didn’t put weight on as quickly as my friends bottle fed babies, they were still healthy and looked far more substantial than the little one here. Their birth weights were both a few lbs lower than Con too.
She needs to get her head around the fact that continuing to breastfeed is not what is best in theses circumstances. She’s failing him. It’s not easy - cluster feeding all day sometimes, but if you want it to be successful that’s what you need to do. Not express just after feeding him or when he needs feeding - it negates the whole point of expressing!