They wouldn't wait a month.
If there is no tumor.. surgery is unlikely.
I would not just jump up in her face and say OMG you are such a liar... but play it cool.
Don't get overly into her story.
Just say, oh wow, that sux, so sorry.
I hope you come out of it ok.
And then just leave it cool, If her story becomes more and more outrageous... or if suddenly she says it was a wild misdiagnosis, then you know it was a lie.