Aselam Aelykum
First off all, different people might respond differently. Mostly depends how insecure the person is, and wheter you say it casually (like it's not that big a deal) or with a certain stress (like it really bothers you). The mint-advice people gave is good as well. Even better is to take one for yourself first, and then offer her one, like it's not because she needs one, but out of politeness. Anyway, perhaps the best option here is not to mention it at all? I mean it's not becuase you let it slide for now, that you'll have to deal with it for the rest of your life. For all you know the person was aware of it, but hadn't had the time to brush or do something about it?