What would you do if you saw the opposite sex friend naked? Would you get a good look at them before leaving? would you feel very awkward? would you leave quickly, or would you freeze in your tracks? How would you feel towards them after the incident?
I've been caught naked before by my guyfriend, coming out of the shower, and he was staring for like 2 seconds (shock I guess) and then he turned around, blushing and apologizing. I felt bad for him lol.
Me, I would just turn around and apologize. I try to be respectful and polite.
Depending on physical attractiveness I would either freeze, look totally shocked, and get a good look, or leave quickly. Then I would apologize, or maybe thank her and then apologize. Attractive or not I would probably feel kind of weird for the rest of the day. I'm sure after a while we would laugh about it.
And obviously if she did it on purpose (and was reasonably attractive) I would stare. I would also probably have a weird look on my face like, "why are you doing this?"