While I would have much less of an issue with a quick #1 change (I've seen parents do this with their kids standing on their legs), a #2 belongs nowhere but in the bathroom. I would think most parents would do it in the bathroom if for no other reason, for the dignity of their child (who wants strangers watching you change a diaper?).

That said, there would have to be a pretty darn good reason for a parent NOT to use the bathroom for ANY diaper change, and that you are in the window seat is not a good reason. If all the bathrooms are physically broken, maybe a #1 change out of sight of the other passengers.