Where do you draw the line ?
The answer is "nowhere".
You have an interpretation of Christian theology that sounds like mine.
So what ?
In any case you're confusing tradtion, doctrine and personal faith.
One core of Christianity is that you have free will to work out what is right and wrong. Different people will reach different conclusions, yet both have the same moral standing, even if one is in error, or frankly stupid. It is the attempt to be better that matters.
I don't have the right to decide what you believe, any more than you have to say her beliefs should be disregarded.
I can't see any plausible alternative to accepting her assertion that it's her faith. She labels herself a Christian, but that is a very wide spectrum of beliefs, and that to my certain knowledge, some large sects require some women to wear crosses.
The only line I see as reasonable is one of practicality. You scoff at the notion of going topless but to a moslem woman, being forced to take off her veil is much the same thing.
It someone wants to dangle something from her necks that might get caught in machinery, then that is valid, but discrimination against small groups is at least as bad as discrimination against large ones.
Her faith is equally valid, even if she's the only one who believes this crap.
Again I uege you to think through, using examples from history, what happens when the state gets involved in deciding which interpretation of which religion is "acceptable".
OK, DC, if you're not prepared actually to consider the theology of Christianity and whether it actually creates a religious obligation... if it really is all just what one perceives as 'religious', then where will you draw the line? What if she thinks her obligation as a Christian or a Rastafarian or whatever it is requires her to go topless? Will you take that seriously as well, simply because she asserts it? Or would you actually look around and say, 'well, nothing in Christianity or Rastafarianism requires you to go topless, so I don't accept that this is an issue of religious freedom'.