Agreed. I like to approach it very tactfully, almost like i don't know what the problem is and agreeing with them that leaving a dirty window behind would be unacceptable. I ask them to look at it together. Then i agree with them that ooo it does look bad! Then i swab it with my finger and act surprised when the dirt is on the inside. "Wow, crazy how that dirt can build up on the inside, huh? Don't feel bad, I've seen way worse than this!"
Usually, though, most people have been happy with how the windows looked even when it was just the outside. If i anticipate a problem, i try to be the one to point it out first. If necessary, I'll even demonstrate polishing with steel wool (steel wool always impresses people that wow, you're really doing everything possible) and that whatever it is won't come off. Same in the case of hard water stains, in which case i let them know there's a chemical treatment they can get if they'd like.