Ruha means either a single dress or clothes, which already is plural. Both make sense here.
Ruhák would here mean that she is not wearing multiple dresses. Which might be possible in another scenario. With an other woman next to her wearing more than one dress and this one does not.
I respect that many people find nothing wrong with this statement but it was not too long ago when it was recognized that this phrase might be seen as inappropriate in some circles. Could we keep this "G-rated" for kids that might be doing this, not to mention some of us "more old fashioned" folks?
I recognize that I live in a world where this is more and more common and I choose to try my best to avoid it without saying too much. However, on a language learning forum, I would rather not have to translate it as an exercise.