Well, first, the same rules of harassment apply to her because she's a goddamn human being, and all human beings, regardless of what they do or do not choose to wear, deserve to feel safe and be free from harassment. Second, the same rules of harassment apply to her because we're human beings, or alien anthropologists who know damn well where the standards for human behavior lie, and once we're old enough to buy convention memberships and book hotel rooms, we should know better. (We should know better before then, too, but that's a matter for our parents.)
I have several friends who do cosplay, both at general science fiction conventions and at larger genre conventions, such as San Diego Comic Con. This means that they are going out in public in outfits they have spent a lot of time and energy making, which may or may not be as concealing as standard street clothes. (One of my friends regularly cosplays Emma Frost. Another has won awards for a Na'vi costume which consists largely of incredibly well-applied body paint.) This also means that they are inviting a certain amount of looking at them: no one puts that much effort into looking amazing when they don't want to be looked at.
Note that "look" and "leer" are not the same word.
What is appropriate? Admiring a cosplayer's costume. Admiring how well they fit the character. Asking if you can take a picture (providing they're not in the middle of doing something else at the time, like say, trying to inhale a hot dog before their next panel, running to the bathroom, or otherwise being a biological creature in a material universe). Asking if you can take a picture with them. Asking about the workmanship that went into the costume's design.
What is not appropriate? "I really love it when a girl with decent tits dresses up as [character]." Trying to take pictures of disembodied lady pieces, like butts or boobs (also inappropriate: disembodied dude bits—they're just rarer). Quizzing them on whether or not they even know who they're dressed up as. (Spoiler alert: anyone who spent ninety hours making a picture-accurate Illyana Rasputin costume probably knows who she is, and if it's someone who, say, joined a group costume to make their friends happy, but prefers DC/spends too much time gaming to read comics/is really happier in the SCA, how is that hurting you any? There is no such thing as a fake geek.) Asking if they'll give you a spanking. Asking how much they charge by the hour. Asking if you can touch them.
There is something magical about meeting a really good cosplayer dressed as one of your favorite characters. They're avatars. Watch a small child meet someone dressed as Iron Man or Aang and see them stare in open-mouthed awe. Hell, watch me meet a really good Tinker Bell at a Disney park. Costuming is a form of magic. It makes the unreal concrete and tangible. It deserves respect.
But those Tinker Bells that I meet at Disneyland have handlers, people who will immediately step in if anyone crosses a line or makes the pixies uncomfortable. What's more, those Disney pixies are paid to be my fantasy, as long as that fantasy remains G-rated and friendly. Cosplayers? Not getting paid. They are people, and they have a right to the ball. They also have the right to say "please take your hand off me" or "please don't take pictures of my ass" without getting told "well, you shouldn't have dressed like that if you didn't want the attention." Wanting attention and wanting to be harassed are very different things.
And as a note: cosplayers are not harassing you by walking around being attractive, or semi-clothed, or interesting to look at. They are not "teases" or "gagging for it" when they put on something skimpy. They are not here to be anyone's private walking skin magazine. They are people.
(Yes, this means they can be inappropriate too. We had an issue at one of the comic conventions a few years ago with someone dressed as Deadpool inappropriately touching female attendees, and then running away. He couldn't be distinguished from the eight or so Deadpools not being giant sacks of asshole. Last year at Emerald City Comic-Con I observed two woman dressed as Jean Gray saying such nasty things about a woman dressed as Emma Frost that she was virtually in tears. None of these things were, or are, appropriate.)
Cosplay makes our conventions more visually arresting. It's a powerful form of expression. It's a hobby and a passion like any other. But costume does not equal consent.
Again, if this is something you can't trust yourself to grasp, maybe you need to stay home.