Not a bad idea, I would think.
Middlebury definitely has a distinct setting, and many seem to think it also has a pretty clear vibe–and she might love it or hate it, so it would be good to know!
And it also seems like a strong ED contender, which is fine if she really wants to ED, but I am always very, very reluctant to suggest a binding application to a school you have not visited if it can be avoided.
Of course Amherst is good too, but I think of Middlebury as one of the strongest possible small-college choices for Environmental Studies (and obviously a great college in general for those who like the setting and vibe). So I would personally think it makes sense to give Middlebury a chance to beat out Amherst for that slot.
By the way, Bates too in my mind, meaning Middlebury, Colby, and Bates are the three Northeast LACs I would first think of when it comes to ES, and then I would also toss in Macalester and Oberlin outside the Northeast. But apparently she likes Colby and Macalester better than Bates, which is fine.
Oberlin, though, might be another to consider for a visit. Even if it isn’t an ED school, it might be worth keeping as a possible RD school, and it would be a potential (I’d guess likely) merit school as well.
Of course many of the other colleges on your list, including Amherst, are going to be good too, because they are good at everything they offer. But for an ED school? I would think normally it would be one of the top few in your area of interest, although if it remained Amherst that is fine too.