Jenna was only partially right - a child's place is is with it's mother and father. In families with divorced parents, children do best when spending equal amounts of time with each parent. If she's strictly talking about infants, I think there's merit towards the child being mostly with the mother, but otherwise, spending equal time with each parent should be the goal. if one parent has some type of a problem making him or her less capable of a parent, such as possibly some sort of a mental health problem or an addiction, then their percentage of time would be adjusted downwards accordingly, depending on the details.
If one parent moves away, like Donna does, she forfeits some of that time, just as if the Ray had moved to Canada, he can't expect Donna to follow him there just because he wanted to move, so Ray would forfeit some of his time.
Donna would have to decide does she want to have her 50% of the time with Margaret, or does she want to move to DC and give Ray primary custody, with Margaret living there with her during summer vacation, and visiting her on some weekends? It's up to her -if her top priority is Margaret, which ideally it would be, she'd stay in Texas and work in local or state politics. If she decides working at the federal level in Washington is of paramount importance, and Margaret is just going to have to deal with getting short-changed of spending half her time with her mother, then that is her right to choose to do that, but then Margaret will spend the majority of time with Ray.