No, I wouldn't say anything to mom
at all. I don't think there is any way that you can put this to mom without her feeling attacked, put down or a plethora of other negatives. I've never saw a parent react well, or in a positive manner, when being told something unfavorable in regards to their parenting.
I also agree 100% that what the girl said may not be all that accurate; I too take everything a child says with a grain of salt, and I would want their parents to take everything their child says about me/my place that way too.
And as someone else said, if it's true, and it sounds like it is at least to some extent, I am sure the mom is already aware of the problem and feels plenty of guilt for it already. So no, I would not say anything; I would though like you said, every time the little girl brings it up, reassure her that her mommy loves her very much and wants to spend time with her, but is just really busy with work & things right now(..and that the reason she's working so hard is to provide a good life for her), and that sometimes in life that's just the way things go
.