I am certain that it would be most difficult for a mother who has been consistently present and active in her child's life - and for the child as well. The bond between a parent and child is a particularly strong psychological development.
[ Various human bonds are of great interest to him, after all. Also of interest is her phrasing when present within the facility - the implication being that her daughter is here. Theoretically, it's possible that she's only referring to her life prior to being brought here, but the way she speaks makes that seem unlikely. He doesn't pry - yet. He certainly will, because he is Data. ]