I'm not going to tell a female co-worker that her jeans zipper was wide open
Why do you not want to tell her?
Don't want to embarrass her.
You should tell someone if there is something wrong with their clothing or appearance. You would want someone to do that for you.
I can see right through her jeans, I am not going to tell her.
You can leave her a note discreetly on her desk that her zipper is down.