I would also think about the commute. There are busses to Thayer and Dexter/Southfield, not sure about Dana Hall. Southfield is more sporty. Thayer kids are involved in something - many are sports, but can be other things. Dana Hall has boarding for HS so it is a different environment. They all have campuses - haven’t been to Southfield but looked at camp there (and have been to British school several times).
Does she want all girls (Dana Hall, Southfield but some connections with Dexter) or coed? How do the size of MS compare? How many stay for HS? How did she feel visiting/revisiting the schools?
Write a pro list for each school and see what is important to her.
Good luck with your decision.