I actually have 4 children, and have had days when I made some tough choices about who to support "most". Like the day my youngest was sent home from school with strep throat...and his sister was graduating HS that night. I missed the graduation--I was literally at the doctor with the little guy when his sister walked across the stage. DD was fine with it.
You seem to be very critical of the concept that this mother wants to devote time to her DD--and also to her son, but now they have to share her time and resources. It seems like this family has a lot going on in a short space of time, such as the graduations, as well. I can understand why the MOG is feeling overwhelmed. She brought up her concerns, and got labelled as "difficult" for her trouble. We also have no idea of the finances involved--perhaps she's promised financial help to her DD, and hoped to do the same for her son, but can't now, due to the close dates. As a mom, that would bother me. I think the MOG had valid concerns/reasons to ask for the date to be pushed out.