By any reasonable measure, that's grounds for termination; her actions harmed the company's reputation and cost them financially. If you can't fire someone for that, I'm not sure what you could fire for.
But no smart company would pursue that route, the PR would cost them more.
See http://en.wikipedia.org/wiki/At-will_employment and specifically the "Statutory exceptions" section.
I guess I wouldn't be surprised if shitty American employment law permits it, but it's not ethical on the employer's part.
Edit: Especially given my short response, I think it is important to reiterate how much I am not a lawyer.
Also in this case the reasons have nothing to do with her gender, but more that she failed at her job at being a developer evangelist.
[1]: http://en.wikipedia.org/wiki/At-will_employment#Statutory_ex...
"Harassing photography" is banned. I'm setting aside the question of whether her Twitter posted photo counts as "harassing photography", but I can't find a blanket ban on photography without permission.
If she violated the standards of conduct, since employment is conditional on compliance, they can fire her and will most likely win any lawsuit on the matter unless a court deems that the standards themselves are illegal.