So, logically, a cis woman who's had a hysterectomy, has not had her genes checked, has a unisex or "boy" name, or who was born with a vagina but an accidental clerical error listed "M" on her birth certificate (all of these have really happened, though some are more common than others)... is a man?
And if she's THAT hung up on these things, I assume she interrogates all women she meets about their internal organs, life history, karyotype, etc.?
Hell, I'd say the easiest way to shut her up is to point out that unless she's actually had the test run - and she probably hasn't, it's expensive and most doctors won't order it without cause - she doesn't know she herself is not genetically XY. It happens, and some women like that grow up with typically female bodies (it's less likely that they'd have a fully functioning reproductive system, but not impossible). So if you turn this around on her every time by pointing out that by her own rules SHE might be a man, she may realizing she's having less fun fast.
