Sexual violence is not acceptable at any time. It does not matter what a person chooses to wear, nobody "deserves" being sexually assaulted.
Whenever you can, make a choice that shows you don't support sexual violence. Listen to your gut, because if it feels 'wrong', it probably is.
Even though you can't always stop an assault, it's never too late to make a difference. And as a bystander, you always have more than one option.