There's no need to bad-mouth your inappropriately dressed coworker. But if you see someone acting selfishly or exploiting people, feel free to pass it on. If you speak up to protect others, it can actually help lower your heart rate, according to a study in the Journal of Personality and Social Psychology.
Your action plan: Since you (hopefully) don't witness selfish acts every day, keep this one in mind for the future. Otherwise you risk crossing into rumor-spreading territory.