Rashi is not trying to prove NDK.
Rashi mentions the story in the context that bruria “scoffed” (anyone have better translation?) at the halacha that 2 women do not act as a shemira for each other when with one man and an issur yichud remains because of NDK. As Rashi explains since they are easily swayed (at least as far as this topic is concerned – see gemara in sota, rotze kav echad etc) each woman will assume the other sinned and will not be embarressed to refrain herself. Bruria disagreed. R’ Meir proved that she could eventually be swayed. Where do you find that R’ Meir “failed” such a nisayon?