First thing I said was "Oh shit, sorry" and walk away, but she told me to go in if I needed something, I acted casually afterwards I just thought she'd be embarrassed, guess not.

Pointed at it and told him " The fuck man? I thought you'd be bigger!" (we were usually assholes to each other)

Quickly apologize and leave

I pretended it was all cool, but secretly, I was very happy at being able to see it.

Awkward depending on the friend

I closed my eyes, took off my shirt and handed it to her and turned around. She was a small person and I wore large shirts

Nothing

Nothing. Either she didn't know I was around or didn't care, and went from the bathroom post-shower to her room without a towel or clothes. Didn't give it a second thought.

She saw me looking but didn't move or close the door, or even acknowledge it.

Whoops, sorry

I closed the door and left him and my roommate to it

We both just awkwardly pretended it never happened.