So what do you do if you see this sort of thing? I would want someone to tell me. But I don't always tell others, even people I know well. When I do mention it I try to be subtle. I've handed a coworker a tissue, told a woman she was trailing TP.
A teacher once told me that she was on the beach with her new husband when he drew close to her and tapped his lip. She leaned in to kiss him and he pulled back and tapped his lip again. She was nose to nose with him when he whispered, "mustache." HIS upper lip was clean shaven. They're still married.
My latest quandry is do I tell the lovely non-English speaking, overweight, grandmotherly neighbor that the new outift she has been parading around in for three evenings is not a matching shirt and capri set? She's wearing a pair of cotton summer pajamas. They are the prettiest print, little blue flowers on white background. She had guests over tonight and she showed off her garden, then strolled out front to show them her fruit bearing trees and the zucchini growing across the lawn. (Yes) I respect all boundaries and cultures, but do I dare say? She really loves her new lounging jammies.