there are many ways to be dan l’kaf zchus the lady for commenting on the tznius violation, and it may be muttar. the extra comment “her relative should be ashamed of her”…
“it is forbidden to say a/t about a person that might embarrass him if it were to be repeated in his presence… unless its for a constructive purpose.” what constructive purpose was there in that? being an opinion, not fact, also makes it hotzaas shem ra. only about a mumar l’hachis is it muttar to focus on his negatives-but still only facts. “however, one who sins out of ignorance or temptation is in the category of amecha and is forbidden to speak l”h about.”
i know, its terrible to quote an English sefer Cofetz Chaim, but its easier to type.