If you knew her and had reason to believe she wouldn't like he bra strap being down, go ahead and tell her DISCREETLY.
When in doubt: DO NOT. Failing to tell her will is unlikely to ever hurt you and, though yes, she may suffer some embarrassment from it, it will be no worse than you wondering around (as Chemflunky said) with your fly down.
The difference here is that you are unlikely to perceive somebody telling you that your fly is down as threatening, but some women are more likely to perceive somebody talking about their bra as threatening.
When in doubt, don't.