That should be easy. All she has to do is prove that some woman told her the story she said the woman told her. In the mean time these asshat professors know that proving their challenge is also proving a negative.
I don’t doubt that the women did indeed approach her and maybe even say what Bachmann said she did.
But MB also gave it credence with her final statement to the press on the incident, therein lies the rub.
Look it up.